I have:
-forgot to plug in heaters
-forgot to turn off heaters
-forgot dechlorinator
-overfilled tank during wc
-overdrained tank during wc
-failed to quarantine
-failed to properly identify pathogen
-overdosed medication
-used wrong medication
-introduced incompatible tankmates
-failed to provide proper diet.
-dropped lights into tank
-failed to provide proper size tank
-failed to maintain healthy water quality.
All of these have resulted in a fatality for me at some point in time. Couldn't help but get down on myself when trying to think of this list, but I would still say I'm a better keeper now than I have been in the past at least.
I was going to say, "well it could have been worse", but on reflection, I don't think it could! Unless they were all in the same day!!!! That would go down as a bad day at the office, lol.