I haven't had any detrimental disasters as of late but I do have one that was a serious WTF moment.
I have a 45 gallon tank at work, which used to house a huge armoured pleco. He was the best fish ever and got to about 30cm in length. I always feed the fish at work in the morning and when I went to feed him I couldn't see him anywhere, the tank was decorated as a "ship wreck" theme, so I thought he was just chilling out inside the ships as he usually did, after not seeing him for 3 days in a row I asked some of my employees if he had died or if anyone removed him from the tank and everyone said that they didn't see anything..
I came to find out that one of my guys that looks after the fish when I'm away changed the water and cleaned the tank.. When putting the water back in the tank he didn't do any tests or add in any dechlorinator to the water, just straight out the tap and into the tank