this one is surprisingly hard to come up with an answer to, because a lot of the most inexplicable stuff is easily explained. even the most wild glitches i've experienced ended up being pretty trivial in the end -- dying hardware resulting in graphics distortion, or other weird issues. or when i was experiencing completely random frame drops, from my motherboard's very specific model being incompatible with a specific cpu. (solution: undervolt and underclock...)
the real one that gets me is when computers turn themselves back on from a screensaver, with no obvious cause.
laying in bed, roll over, and oh, my desktop's screens are on again. the mouse didn't move, no applications seem to be calling for attention, everything is otherwise as it was ... the screensaver just turned off (and sometimes refuses to turn back on).
to this day i still can't find an explanation for why this happens, and it's not just any single computer but something that's gotten me across my desktops for the last several years.
it's largely stopped, but every few weeks it'll do it again.
