Your dumbest reef related move? Here's mine....

[fishoholic]

I once left my automatic top off hose just resting on the edge of the tank, normally held into place with a zip tie. The hose fell into the tank, back siphoning water from the tank into my RO top off container then over flowing from the container onto the floor.

I had a 6 bulb T5 light over my QT tank, it rested on two pieces of wood that sat on top of the tank. Had finished with the QT tank (a few weeks ago, at the time) but hadn't got around to cleaning the water out or putting the lights away. Came home after work and found the T5 lights inside the tank of water, no idea how long they had been in there for, but needless to say the lights didn't work after that