I had lots of fun with that scenario. "My mechanic says there's something broken in my transmission", I'd say, "He didn't put that back in? Does your transmission still shift okay?"... Shit like that. Good times.
A previous dealership I worked at, had one of those "wallet flush" transmission service machines. I hated using that machine. I prefer to do a drop the pan, change the filter and fluid type of service. You'd be amazed at how many pans I had removed with 200K + on the odometer, that have had several "transmission flushes", but never had the pan removed once, and running on the original filter.