Hmmm, well there was that one girl I met at a party. We got pretty hammered and then we went out and.........
OH, you mean fish related mistakes...
I don't know, I've always had very good luck overall.
I guess one that sticks out is when servicing a tank in doctors office once that had three of these guys in it;
turns out their spines are poisonous and generally dirty/nasty and when trying to get one to move out of my way, he tagged me in the hand.
Hurt a little, but within an hour I was sick as a dog, a very sick dog, my hand swelled and I was ill for 3 days. It was actually worse then the two times I got tagged by lionfish.
My mistake was not being familiar with the fish.