I did the dumbest thing I've ever done last fall, on a house I own but don't occupy yet. It didn't really turn cold enough for the furnace until November, when I went over the get the 40-year-old thing running again so my pipes didn't freeze. The heating system uses hot water and radiators and periodically needs to have the water flushed out to work properly. I opened the stopcocks but nothing came out right away, so I closed and re-opened them a few times and then forgot which positions were open or closed. I then went home and it turns out I had left the cocks open.
Came back the next morning to ten inches of water in the basement. I have a sump pump but I had forgotten to plug it in. Had to walk barefoot through freezing cold water to plug it back in and at least it emptied out the basement in about 45 minutes. My front yard was hilariously soaked.