Well, I have 3, if I restrain myself.
Early in my fish keeping, I decided to start a 10gal tank in my college dorm room. The room was small and 'very limited' on stable surface area. I had a roommate, we shared a long desk(sitting side by side), with two long shelves above. My roommate had an Apple Macintosh Computer (yes, it was that long ago) on one end of the desk, so we didn't have much space to do much else.
Finally, while he was in class, I decided to set-up the tank; I didn't care/know about cycling or prepping the tank in any way. I filled the tank, bought the fish, then realized it was too big to put on the desk - and actually sit there. I measured the shelf above and it was a perfect fit, with maybe a half inch to spare. I emptied the tank about half way, placed it on the shelf, and refilled. During the refilling, the tank shifted about an eighth of an inch. I'm a bit OCD, so I tried to shift it straight. The shelf was buckling under the weight of the tank, and when I touched it, the back glass (I still thank God it wasn't the front glass) cracked from one corner to the opposite corner, and the back of the tank exploded. Spilling 10gal of water, glass and about 10 fish onto the desk, computer and carpet below. My roommate moved out.
Second one; While moving to a new apartment, I put my Pacu in a small cooler for transport. While waiting to set-up the tank at the new place, I forgot I had put the heater in the cooler to get the temp back up to what the fish were used to. By the time I remembered, they were all cooked and floating. No longer delicious.
Third; After keeping fish for nearly 20 years, I was doing a water change and got side tracked with the tv. While refilling the tank 1 pitcher at a time, I forgot to check the water level. I "over-filled" the tank.
