I've done the "sleepy, tired, and want to go home" not thinking very much and checked in with United on a US Airways flight. Reservation was United, outbound was United, day was long so on the way home I walked to the United check-in area. Got the boarding pass and checked in my luggage.
Walking to security to see what gate to go to, that's when I finally notice it was a US Airways boarding pass. Me and the luggage still made it.