Intermittent failures.
First half of my career was as a field Tech on analog photocopiers. The dreaded "intermittent failure" was always the bane of a Tech's existence.
When dealing with an intermittent problem I was unable to duplicate, I never really knew if that last thing I did fixed it or, if it just went away intermittently, until an indefinite period of time had passed.
One thing I learned from dealing with intermittent issues was, to believe the end users who were telling me what the symptoms of the failure were, however non-technically they phrased it.
After all, they don't know the equipment well enough to make up what they're saying.
slmjim