I'm sure your vet would make absolutely positive that your cat had passed on before telling you so. I don't think you have anything to worry about there.
I've seen two cats be put to sleep. My parents' cat Lucy had a closed mouth and she had definitely passed on; I was the one who buried her so I know for sure she was dead. Bentley's mouth was open slightly when he died, but it was already open due to the seizures he was having, so I think that was not from the euthanasia.
My guess is that Misty looked peaceful because she was peaceful - she was beyond pain and fear, so it really was just like going to sleep for her.
The Boys: Maisie, Zephyr, and Cullen.