A couple months ago I had the flu and took a decent amount of Nyquil to get to sleep. I woke up vaguely remembering sleep walking to the fish tank. I went over there and saw I'd tried pulling the filter off, knocking off the intake thing, leaving just the tube. As soon as I turned it off to see what made it's way up there, 5 very decimated fish came rolling out. It was only my guppy tank so the losses weren't severe, but I still don't know why I felt it necessary to attempt to remove the running filter off of my tank while asleep...
ETA: I almost forgot an incident I had when I first started keeping fish. It was a tank I had bought off of craigslist - 20 gallon, with an insane amount of jewel cichlids. Everything surprisingly went fine until one morning I woke up and the water was dark brown and every fish was dead. It ended up being that one of the jewels rammed the heater and broke it, which caused them all to boil to death. Worst part of it is I'd woken up in the middle of the night and noticed the water seemed kind of murky but didn't think to check it out/do anything about it.