Can someone please help me with a one word substitute for something (an event probably) that happens only under very specific (and strict) circumstance?

So, my firm is a software vendor. There is this client who uses this software. They faced an error when they used the software in a very specific way. Or when they specifically followed a set of actions in a certain sequence. Now, they are afraid that they might encounter this error on a regular basis.

What I want to convey to them is:

This was a/an _______ error. Which you encountered because you specifically chose to use the software the way that you did in this particular instance. In future you will only get it if you again follow the same set of actions in the same order.

  • 1
    You may consider that unparalleled is suitable, though I wouldn't say it is a precise fit. – Edwin Ashworth Feb 14 '17 at 8:51
  • Rare? Unusual? Controlled? Orchestrated? – Lawrence Feb 14 '17 at 8:58
  • @edwin-ashworth, Thanks, but no. "unparalleled" does not suit my context. And sadly, it's quite difficult for me to explain my context also. :( I am looking for something probably along the lines of "difficult to recreate/reproduce". – Shubham Sharma Feb 15 '17 at 3:25
  • @Barmar, edited to add details. Please see if it makes any sense now. :) Cheers! – Shubham Sharma Feb 15 '17 at 3:37


A "one-off" is an event that occurs with no frequency or regularity. The term is more popular in Britain than some other places, but is in general use. It can be an adjective or a noun.

Landing marines on Cyprus was a one-off.

Landing marines on Cyprus was a one-off event.

As most words in English, "one-off" can be used in a variety of contexts. But in common usage the word does convey the intention stated in the question.

  • He says "circumstances" (plural), which implies that while infrequent, the event may repeat. A "one-off" is generally expected to never repeat. – Barmar Feb 14 '17 at 20:19
  • The OED says:"Something done, made, or happening only once, not as part of a regular sequence.". Other sources claim a similar lack of regularity. I understand your concern about the word, I often hear it used to mean infrequent or irregular, in addition to singular, – J. Taylor Feb 14 '17 at 20:28
  • @J.Taylor, Thanks for your answer, but I agree with Barmar. I am aware of "one-off". But it restricts the frequency to strictly once ever. That is not what I intend to convey. Like I said earlier, " I am looking for something probably along the lines of "difficult to recreate/reproduce".". – Shubham Sharma Feb 15 '17 at 3:28

A term that can be used is rare parlay. I can only find the literal definition related to gambling in dictionaries, e.g. from ODO:

A cumulative series of bets in which winnings accruing from each transaction are used as a stake for a further bet.

But in my experience, the word is often used metaphorically to refer to any situation where you only get a particular result from an unlikely combination of actions and/or circumstances.


Extremely-hard-to-reproduce software bugs are known in the trade as heisenbugs. The term incorporates the notions of "very specific (and strict) circumstance[s]" and "difficult to recreate/reproduce" that you want to evoke.

In computer programming jargon, a heisenbug is a software bug that seems to disappear or alter its behavior when one attempts to study it.1 The term is a pun on the name of Werner Heisenberg, the physicist who first asserted the observer effect of quantum mechanics, which states that the act of observing a system inevitably alters its state. - wikipedia


To follow up on the original-post comment by @1006a, you may want to consider the effect of suggesting that this kind of error occurs often enough to warrant a specific name: "Oh, that was an x error. (We've seen those before.)" Also, this type of language may cause your client to believe that you're trying to minimize their concern.

A better approach may be to provide the client with a concise description of why the error occurred, why the problematic configuration constitutes an unsupported use of your company's software, and how they can prevent it from occurring again.


Instead of referring to the odd result as an error, call it an anomaly, a strange occurrence—a mystery:

  1. Deviation or departure from the normal or common order, form, or rule.
  2. One that is peculiar, irregular, abnormal...

American Heritage® Dictionary of the English Language, Fifth Edition

  • Thanks @Yosef, but I know what an anomaly is and this was not one. An anomaly cannot be predicted and cannot be guaranteed to recur even if given the same initial conditions and procedure as the first time of its occurrence. I think we might not even be aware of the initial conditions of the system and the procedure that led to the anomaly the first time. This is my thinking, let me know if you differ. Cheers! – Shubham Sharma Feb 17 '17 at 10:32

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