It’s extremely rare. Veterinary facilities will have different ways of doing things, and it actually varies a lot, from state to state. It’s pretty hard to do. The euthanasia drug, is a controlled substance. So. Only certain people can even get to it, let alone administer it. In my state, the is a log, that must be filled out, with owner information, and addresses etc. So. You’d really have to drop the ball, to have an accidental euthanasia. The real concern, is that the animal that is set for euthanasia, is actually owned by the person paying for it. That’s hard to prove. But neighbors will bring in a barking dog, or cat they deem a pest, and say it’s their’s and try to have it put down… Gotta love our fellow human beings…...