The mouse doesn't work in the afternoon..
Back in the 90s I was working at a university, there was an office belonging to some affiliated organisation that we didn't really understand which we were apparently responsible for supporting.
One day I was in the office doing something or other, and the staff mentioned that they couldn't use one of the two computers in the office in the afternoon. I asked what they meant and they said it just didn't work, they seemed happy to have the afternoon off and I suspect they didn't do much work in the morning either.
Some further enquiries revealed that the computer would boot up into Windows 3.1 (yay) but they couldn't control the cursor. Worked OK in the morning, but on many (but not all) afternoons the mouse stopped working and the pointer wouldn't move.
Back in those days we couldn't Google stuff, but I did reach Jerry Pournelle's "Chaos Manor" in BYTE. Jerry had mentioned a similar problem, so I took the mouse apart and found the problem - this early opto-mechanical mouse didn't have a housing around the optical sensors. In the afternoon, sun came through the west-facing window onto the mouse and flooded the sensors. If it wasn't sunny, then the mouse would work.
Unlike today where most IT departments have a cupboard full of spare mice, we didn't and had to order one (I think it was about £25) and wait a couple of weeks. Installing the new mouse (with the internal housing) fixed the problem.
Weirdly, the users weren't happy about the fix. The whole organisation was shuttered a few years later, can't think why..