Well hello everyone, I'm pre-dent but have been a vet tech for 5 years so I have some pretty good stories. Here's my favorite all-time worst moment ever:
We euthanized a fat old cat and had it tagged in the freezer for a private cremation but the owner changed her mind and decided that she wanted to take it home and bury it instead. Well, the cat had been in the freezer for a day so it was rock solid frozen, but she wanted to take it then anyway. So I went and got it out of the freezer and went to put it in a box, but she insisted on me just taking it out to her car in the garbage bag so I complied and followed her out to her car holding a garbage back with her hugely obese frozen cat in it. Well, of course, the trash bag broke, which is bad enough, but then the freaking cat BROKE when it hit the pavement! I am not even joking, its freaking paw came clear off, I have no idea how but needless to say me and the owner both lost it. So now there's 2 pieces of a dead frozen cat in the parking lot in front of tons of people. Oh man it was a disaster. Anyway, we switched to thick cadaver bags after that but the damage was done.
Anyway, letting dogs bleed out, overdoing fluids, etc., aren't very big mistakes, don't sweat them. As an example of a real screw up, a shelter nearby euthanized a microchipped dog 10 minutes AFTER calling the owner to tell them their dog was at the shelter, big mistake by someone. Needless to say, the manager lost her job, the owners sued, and the shelter ended up closing down from all the negative publicity.