I've done this plenty of times. At my parents I put my keys in my purse but left the purse on the seat. In my Cavalier it was habit to lock the door when I got out.
Luckily with my Jetta it's "almost" impossible to do. I cannot lock the doors with the drivers door open. But I do lock the doors while I have another door open. So I could leave the keys in the back seat and lock them in that way. But I also have a valet key in my purse.

But I did one of the most horrible things. I locked my niece in my SIL's truck when she was about 9 months old. She is almost 18 years old now and we can finally laugh about it. She loves to bring it up. LOL