First, came back from a month in Las Vegas to find that my friend who was feeding my fish didn't feed them more than once a week. Tank was a 20L with 8 small african cichlids in it. My friend went by the day before I got home to "Make Up" for not feeding them more while I was gone, He put a months worth of food in at once. Needless to say, fish were dying when I walked in. Tried to start an emergency water change, bumped a rock, rock fell, broke front pane. Water started to trickle out. I told my wife to grab and end, we started to carry the tank outside. We make it out the front door where my wife couldn't hold it anymore. She dropped her end and the tank exploded. I had fish flopping in my flower beds. Managed to save about 4-5 of the fish though.
My favorite was when I was deployed to Qatar in 2003. I traded in my fish from my 2 55g tanks and just turned them off with water in them. Water was gross so I told my wife to just dump some bleach in them and turn on the filters to start to clean the tank. I figured I would just start all over again. Well, we didnt have liquid bleach so she dumped in Tide w/bleach. When she started the filters everything just bubbled up all over the place and made a huge mess. Took a while to clean but it was worth it because I still laugh at it.