For example, the car mechanic can't replicate the problem you are having every day, but when you drive it off the service dept, there it is again. Or, when seeing the dentist, the tooth ache goes away, and comes back when you leave.

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    This is the "my car is making a funny sound" phenomenon. The sound becomes impossible to reproduce for as long as a mechanic is on hand to examine the car. The solution to the problem, in fact, is to make the mechanic travel with you whenever you use the car from then on.
    – Sven Yargs
    Commented Aug 3, 2015 at 21:33
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    possible duplicate of What word should I use for something that fails intermittently? Commented Aug 3, 2015 at 23:37
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    Just for fun: I call the opposite "demonstration mode" in real life. Things that go wrong when you go to demonstrate your work. Commented Aug 5, 2015 at 12:08
  • I call it the "car mechanic syndrome" or the "repairman syndrome" (which was posted as an answer yesterday). Commented Aug 5, 2015 at 20:40
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    "Murphy's law" states that if something can go wrong, it will. Of course that covers a wider range of situations, so it doesn't answer your specific question ... but I've heard "Murphy" invoked in scenarios like you describe. This may be an Americanism or even regional usage though, I'm not sure.
    – David
    Commented Aug 7, 2015 at 18:02

16 Answers 16


Around my office, such a problem is called a Heisenbug, a pun on the name of the great physicist Werner Heisenberg, who first described the observer effect (the rule that observing any phenomenon will change it) and the uncertainty principle (the rule that you can know either where something is or how fast it is going, but not both).

The frustrating thing about a Heisenbug is that there is no way to know for certain you have fixed it. You may believe you understand the cause of a particular Heisenbug, you may address that cause, and the Heisenbug may appear to go away, but you never know..

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    I like "Heisenbug" but I don't think it's very common outside the developer community. I doubt many doctors or mechanics would understand what it meant.
    – Kim
    Commented Aug 4, 2015 at 3:08
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    The word certainly isn't but the name Heisenberg has been widely known in the last few years for some reason. I am trying to enough use of the word, which I think is useful and evocative. Commented Aug 4, 2015 at 7:53
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    Came to say "heisenbug."
    – waldrumpus
    Commented Aug 4, 2015 at 8:24
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    Agree both with the fact that it's an awesome name for the phenomenon, and also that it's widely known among software developers but not generally by people in other fields unless they hang out with software developers. (That said, a heisenbug strictly speaking is one that disappears when you try to study it - in your example, the mechanic would be able to reproduce the problem, it would just disappear once he opened the hood to look for it.)
    – neminem
    Commented Aug 4, 2015 at 20:09
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    Notwithstanding all the upvotes, I don't see how this answer will help the OP. Commented Aug 4, 2015 at 20:40

I would call it an "elusive" problem.

elusive adjective:

1: tending to evade grasp or pursuit

2: hard to comprehend or define

3: hard to isolate or identify

(Merriam-Webster online)


An intermittent problem.

  • stopping or ceasing for a time; alternately ceasing and beginning again. TFD

  • starting, stopping, and starting again : not constant or steady. MW


  • "My new car has been having an intermittent battery problem."
  • "The forecast is for intermittent rain."
  • "The patient was having intermittent pains."
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    Intermittent is good, but it doesn't cover "only disappearing when you're trying to fix it". I think combining this with James's answer to give an elusive intermittent problem would cover the full meaning that the OP wants.
    – AndyT
    Commented Aug 4, 2015 at 8:11
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    I prefer sporadic. Has the same general meaning, but implies less regularity. E.g.: My car has intermittent wipers, that come on and off with a very predictable pattern. If they just came and went at random, you'd call them "sporadic wipers" (my old car had exactly that, due to faulty electronics.) Either way, though, both are missing the "disappearing when you try to fix it" connotation. Commented Aug 4, 2015 at 13:18
  • @DarrelHoffman however, when you've tried to track down intermittent faults, how often have you been able to make them happen when you're ready to address them. The word intermittent may not carry this connotation but intermittent bug/fault implies it IME.
    – Chris H
    Commented Aug 5, 2015 at 9:15

Sounds like a gremlin

An imaginary mischievous sprite regarded as responsible for an unexplained problem or fault, especially a mechanical or electronic one: a gremlin in my computer omitted a line

Oxford Dictionaries Online

For a longer discussion, see this article in Wikipedia

Also, there is a wonderful dramatization of the gremlin effect in aviation in Nightmare at 20,00 Feet, a Twilight Zone episode with William Shatner (Galloping around the cosmos is a game for the young. - Captain Kirk).

  • If you like the word gremlin and you like children's cartoons about Hitler, watch Russian Rhapsody and listen to the Gremlins from the Kremlin sing "We're here, we're there, we're in the Nazis' hair." Commented Aug 3, 2015 at 22:57
  • @AndyT I agree that the term can be applied to a single or constant occurrence of an unexplained problem, but often the gremlin quality is invoked because of its elusiveness in time not just causation.
    – bib
    Commented Aug 4, 2015 at 12:02

I happen to like Repairman's Syndrome, where the presence of someone in the know makes the thing work.

  • 1
    A more specific but possibly more common form of this is "Mechanic's Syndrome".
    – KeithS
    Commented Aug 5, 2015 at 21:25
  • Some years ago, I had the indicated problem; people who called me for technical assistance with things I'd set up for them would often say the thing had a "[myname] Sensor" which would make it work in my presence.
    – supercat
    Commented Aug 6, 2015 at 21:46

A Singing Frog

I call this a "singing frog," and apparently I'm not the only one. The name comes from an old cartoon about a man who finds a frog which sings and dances, but not when anybody else is watching.

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    ♫ ♬ Hello, my baby. Hello, my honey. Hello, my rag-time gaaaaal! ♫ ♬ (It has been decided, for some reason, that the name of the frog is Michigan J. Frog.) Commented Aug 5, 2015 at 18:42
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    Ha Ha, that's awesome! I'm going to start calling them "singing frogs" also. Thanks for the memory. Why can't they make cartoons like that anymore?
    – James
    Commented Aug 5, 2015 at 23:15
  • Check, please!
    – Mazura
    Commented Aug 6, 2015 at 0:07
  • At my school's IT department, there was a sign explaining "Michigan J. Frog Syndrome". I wasn't sure if that was actually a thing. Commented Aug 6, 2015 at 0:12
  • Totally familiar with the frog cartoon from when I was a kid, never realized he had a name, and calling this phenomenon "A Singing Frog" is perfect.
    – delliottg
    Commented Aug 7, 2015 at 16:13

Not an exact match to the definition posed in the question, but it is closely related:

When you are using a device or system and it malfunctions, and then you try to cause that malfunction again to show someone, but it does not malfunction this time, you would say the malfunction is not reproducible.

"not reproducible" does not necessarily mean the malfunction will or will not happen later; it just means it does not happen when you try to cause it to happen.

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    As a slight variation on that, in programming contexts I often see discussed "non-reproducible errors."
    – josh314
    Commented Aug 4, 2015 at 2:19

"Mechanic's effect" is the only term I've ever for the behavior you describe. Oddly enough, a quick search didn't find any hits other one over at "what's the word".


The first thing that comes to mind is:



ˈwakəˌmōl/ noun NORTH AMERICAN

  1. an arcade game in which players use a mallet to hit toy moles, which appear at random, back into their holes.

"next time you are near a kiddie amusement park, go in and play a round of whack-a-mole"

  1. used with reference to a situation in which attempts to solve a problem are piecemeal or superficial, resulting only in temporary or minor improvement.

"the site's security team has an ongoing battle against spammers, but it's a game of whack-a-mole"

enter image description here

  • My thoughts exactly!
    – Charon
    Commented Aug 6, 2015 at 13:15
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    I think this term is not suitable when it is the same problem which reappears. Whack-a-mole would apply if shortly after fixing the first problem a new problem appeared, and it is quite likely the second problem was a latent problem which happened to be exposed by the act of fixing the first problem. It is unknown how many times this cycle may repeat.
    – kasperd
    Commented Aug 7, 2015 at 9:26
  • @kasperd The problem is always a mole. Thus, Whack-a-Mole is a good analogy. You're free to offer your own answer.
    – Wad Cheber
    Commented Aug 8, 2015 at 12:21
  • @WadCheber There would be no point in me adding an answer, as it would end up looking like I had just copied some of the top-five answers.
    – kasperd
    Commented Aug 8, 2015 at 13:05

I've heard a problem like this referred to as "shy," particularly in the case of technical support. A "shy" bug is one that never shows up when the tech is on-site, only to reappear as soon as he or she leaves.


An evasive or a slippery problem. Perhaps even cat and mouse



  1. tending to slip from the hold or grasp or from position: a slippery rope.

  2. likely to slip away or escape: slippery prospects.




  1. elusive or evanescent.


Cat and mouse


b : a contrived action involving constant pursuit, near captures, and repeated escapes

(Merriam Webster)


More technical term:

Unreliable test conditions

In my mother tongue exists an idiom translatable to English as

Natural perversity of inanimate objects

I am aware that it may be applicable in less specific situations (any Murphy law at least), but I'd like to share my idea anyway ;)


I've always heard it referred to as "who-me syndrome." As in "who, me? Problem? No, of course I don't have any problem! See, everything's working juuuust fine!" And then the machine snickers evilly as the person able to fix it shakes his head and walks away.



- a puzzling or inexplicable occurrence or situation
- a person, thing, or situation that is mysterious, puzzling, or ambiguous

- someone or something that is difficult to understand or explain

"This is a complete enigma. I've been to the mechanic three times and they hear nothing. But whenever I'm alone in the car the rattling returns." ~ 20-30 million people a year


Phrase the mechanic says to you:

Can't fix it if it ain't broke.

  • Hi, Mazura. This response is at least as good as the one that I left as a comment under the poster's question; but like mine, it dosn't really answer that question. Please consider converting it into a comment.
    – Sven Yargs
    Commented Aug 4, 2015 at 0:07
  • @SvenYargs "My car is making a funny sound" is not an acceptable substitution for "Can't fix it if it ain't broke". "An intermittent problem." is a term for the situation; not a colloquial phrase. Does it not, therefore even more, directly answer the question?
    – Mazura
    Commented Aug 4, 2015 at 0:15
  • Ironically, originally this was (and still appears as) a comment. I thought it worthy of being an answer. If the OP doesn't find it useful, then they should be deleted altogether.
    – Mazura
    Commented Aug 4, 2015 at 0:31

It's a "Shy" Problem or symptom. Like when you went to your doctor and the symptoms mysteriously subside or completely go away (usually temporarily) or when you get called in to solve a computer, technical, or mechanical problem, and the problem or symptoms cannot be brought forth. Also called "white coat shy", "technician shy", or simply "a gremlin". However, especially in software, it's "an undocumented feature".

  • Welcome to ELU. Please read all existing answers to a question. Shy and gremlin have already been suggested in prior answers (which is probably why you have a downvote).
    – Andrew Leach
    Commented Aug 7, 2015 at 6:38

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