What do you call a bug that occurs once and hardly occurs again or you are not able to reproduce it?

Some "mysterious" bugs occur once, and then they either never occur or occur after months or years. You are not able to reproduce them because you don't know why they happen. Is there a technical name for such bugs?

Solution

This kind of bug is usually referred to as a Heisenbug.

What do you call a bug that occurs once and hardly occurs again or you are not able to reproduce it?
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