Learn English – One word for, something (an event probably) that happens only under very specific (and strict) circumstance

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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.

Best Answer

One-off

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.