Turned off sump system and pulled u-tube out of overflow box the day I went on vacation:
1. Did not realize one of my clownfish flipped a Nemo and was in the sump when i left...no heat...no light...still water...dead fish
2. Did not realize the u-tube linked up a couple places with saltwater creep...water creeped over u-tube, up on top of tank, damped the canopy, and dripped saltwater down the side wires onto the electrical chord, outlet, and carpet. It felt very damp when I returned home, but didn't notice until i removed my socks and felt the damp floor. Thankful there was no electrical fire!
Lost a kole tang when I used a high powered powerhead to circulate water. The only thing it really pushed was the kole tang against a rock. I saw him early in the morning, but he was pretty much wasted away that evening when I noticed this thing crushed up against a rock. I felt soooo bad.