Manual Transmission - Cracked Drain Plug Housing

I would think the plug or the threaded hole would strip out not crack it.?

Yes they probably used an impact wrench on it and cracked the case. You would have to prove it, although common sense would say that if they were the last ones to service it and did not say anything about a cracked case, they more than likely did it.

I wouldn't make either assumption.

First I doubt a complete idiot wouldn't use an impact gun on one.
Second, A tech knows that somthing like that would come right back. I'm not saying that it did or didn't happen, I just wouldn't jump to to a wrong conclusion.

We had a girl come in for inspection, we did the inspection, she left. An hour later she is screaming bloody murder how we did somthing to her car and her radio or somthing wasn't working since we did the inspection. We look at it, get a wiring diagram, find the cigar lighter is on the same circuit. The lighter is in a area where the ashtray is. We look in the lighter socket and there is an earing in there that shorted it and blew the fuse. Turns out she took off her earrings when she came for the appt. And one dropped in the socket.

I would not have mentioned it if he was at a dealership, he wasn't.
I am not suggesting for a fact that they did it, but again, if the owner did not notice it prior to the service, but did immediately afterward, it would seem logical they overtorqued it.