I'm trying to describe software bugs that are hard to track down due to the mistake (introduced by the developer) being a very subtle one.

"when these errors manifest they do so in a ...(pernicious?)... way"

It's on the tip-of-my-tongue and is a word most people have heard of (so it can be used in marketing material without a fear the reader will need to lookup the word).

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    Insidious ? : Working or spreading harmfully in a subtle or stealthy manner: insidious rumors; an insidious disease.
    – user66974
    Commented Jul 21, 2015 at 14:44
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    Since the context is one of "subtle" manifestation of errors, it's pretty obvious there are negative connotations (though to my mind, "subtle" itself is a neutral term). But the problem is most negative adverbial usages of "subtly" (or synonyms) involve additional anthropomorphization (i.e. slyly implies deliberately being subtle). It therefore makes a difference whether OP wants to characterize the behaviour of his errors as intentionally trying to avoid detection, or simply that they're difficult to identify because they're not easily noticed. Commented Jul 21, 2015 at 15:05
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    haha I thought it probably would be! That kinda rules out several suggestions here. Your best bet might be inconspicuous or synonyms, and forget about the "negative associations", since they're already implicit in the overall context. Commented Jul 21, 2015 at 18:05
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    I agree with @FumbleFingers here. There's nothing particularly malicious about incorrectly software behavior that's not easily spotted (It's a computer program with mistakes in it; there's no hint of malice.), so "subtle" works better in a technical context than the more flowery options presented.
    – jpmc26
    Commented Jul 22, 2015 at 15:11
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    A lot of the answers here include a measure of how bad the effect or 'intent' of the bug is rather than just it's difficulty to track/detect. Of the answers, Hidden, Elusive or Obscure are probably best at not colouring visibility with outcome. Commented Jul 23, 2015 at 11:22

11 Answers 11


Insidious is probably your best bet,

proceeding in a gradual, subtle way, but with very harmful effects.


but you might also consider treacherous, which has the same connotation of remaining hidden and doing damage unexpectedly:

1. guilty of or involving betrayal or deception.
2. (of ground, water, conditions, etc.) presenting hidden or unpredictable dangers.


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    It would probably be helpful for you to include a definition for those words as well as citing the source. Commented Jul 21, 2015 at 15:44
  • from Google: insidious: proceeding in a gradual, subtle way, but with harmful effects. Commented Jul 21, 2015 at 18:32
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    As a software developer, I use the word insidious for this exact situation.
    – Matt
    Commented Jul 21, 2015 at 21:06
  • Excellent. I just learned the usage of the word of the movie title. (y)
    – Ely
    Commented Jul 21, 2015 at 22:32
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    Pretty sure you only want to use "treacherous" in cases of actual sabotage.
    – March Ho
    Commented Jul 22, 2015 at 1:13


difficult to find, catch, or achieve.


Or, more fun and more negative:


1. furtive; sly.


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    +1 Actually 'sneaky' works well when talking about software bugs :-) Commented Jul 23, 2015 at 11:21

Obscure is a possibility

hard to perceive

not readily seen

inconspicuous or unnoticeable

However, this would work better if it's a bug that's difficult to reproduce, rather than a bug which is difficult to find the cause of.

Ref: dictionary.reference.com



One adjective that comes to mind is " pernicious ".

Pernicious : having a harmful effect, especially in a harmful or subtle way.

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    Pernicious is in the question. Perhaps that's why it came to your mind...
    – ErikE
    Commented Jul 23, 2015 at 17:44


Surprised no one has mentioned perfidious.

adjective 1. deliberately faithless; treacherous; deceitful: a perfidious lover.

Treachery implies subtlety.


I think you answered your own question - pernicious is a great word for that.

Elusive doesn't really ring negative, nor does sneaky without more context. Insidious and treacherous are definitely both negative, but I'm not sure I'd want to describe even the worst bugs as either (at least not in front of stakeholders).

There are plenty, though: ruinous, nefarious (a personal favorite), deleterious.

  • Hrm... pernicious has a connotation to me of being persistent and hard to resolve despite repeated efforts, though. Not quite what OP is describing.
    – hBy2Py
    Commented Jul 21, 2015 at 19:59
  • Sneaky is not negative without more context?! It's very much inherently negative to me. I can't even think of any contexts where it wouldn't be negative. Commented Jul 23, 2015 at 11:27
  • Talking about kids or pets, it's almost alway light-hearted. Or if someone is sneaky and organizes a surprise birthday party. I don't know, there are probably quite a few ways it isn't negative. Commented Jul 24, 2015 at 0:29

indiscernible or undetectable - in the context you're looking for.

'under the radar' (not a single word) or stealth(ily)

elusive, insidious


Devious (www.dictionary.com)


  1. departing from the most direct way; circuitous; indirect: a devious course.
  2. without definite course; vagrant: a devious current.
  3. departing from the proper or accepted way; roundabout: a devious procedure.
  4. not straightforward; shifty or crooked: a devious scheme to acquire wealth.


adj. Obtained, done, or made by clandestine or stealthy means.



Sly could be used. Here's the first definition of sly in Oxford Dictionaries Online:



1 Having or showing a cunning and deceitful nature

  • “Google” is not a citable reference!
    – tchrist
    Commented Jul 23, 2015 at 13:14
  • Hi, Thomas Molloy. I added a link to the source online dictionary for your quotation and identified it in your answer, and I removed the attribution to "Google," which (as tchrist emphatically points out) is not a citable reference. I think you'll find that identifying and including links to your reference sources will make your answers stronger and hence more popular with upvoters.
    – Sven Yargs
    Commented Jul 24, 2015 at 4:45

I suggest lurkingly. Here are the first three definitions of lurk from Dictionary.com:

  1. to lie or wait in concealment, as a person in ambush; remain in or around a place secretly or furtively.

  2. to go furtively; slink; steal.

  3. to exist unperceived or unsuspected.

  • Is 'lurkingly' really a word? Is there a reference you can point to? Commented Jul 23, 2015 at 8:22
  • Why not? Lurking is a valid word. Just make an adverb out of it. I am not too uptight or by-the-dictionary about these things. dictionary.reference.com/browse/lurk?s=t
    – ArtforLife
    Commented Jul 23, 2015 at 15:19
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    @ArtforLife When quoting, please indicate the source; otherwise, by policy, they may be deleted as copyright violations.
    – choster
    Commented Jul 23, 2015 at 15:55
  • I added a link to the source of the definitions in your answer, but I think that the answer still has a bit of a problem in that it presents an adverb (lurkingly) as the suggested darker version of an adjective (subtle). Perhaps you would do better with lurking as your suggestion? (I think it might be a pretty good match for what the poster is seeking, by the way.)
    – Sven Yargs
    Commented Jul 23, 2015 at 19:40
  • Thanks for the edit and the tips. It is up to the writer to choose "proper" words that he thinks relay his meaning best. I just suggested this as an option.
    – ArtforLife
    Commented Jul 23, 2015 at 19:45

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