I sent my Commodore 64 for repairs more than once only for them to find nothing wrong until we realised it was the buildup of static electricity from being stored under our big TV - it "broke" when we didn't use it for a while but watched lots of TV. It'd be "fixed" by the time it took before the repair people got around to it.
The symptom was that it started "typing" automatically.
I had a weird 'vibration' in the monitor for my first PC (this was after I caused it to decolor it by passing a speaker too close by it); took it to the shop, no issue. Back home, issue. I believe it turned out to be a power brick that was too close to the screen or its cables.
The symptom was that it started "typing" automatically.