Some years ago I checked into one of the Marriotts in Palm Springs for a convention, and decided to take a shower before dinner. While I was drying off, I heard the room door open; there stood another conventioneer who had been assigned my room! We decided I'd been there first, not to mention he had more clothes on, so he went back to the front desk and got a different room.
After dinner my (electronic) key wouldn't work - his check-in had negated my key's code, so now it was my turn to go to the front desk and explain the problem.
And although the front desk must have heard twice about their mistake, no free night, not even an apology...but after reading on this board, I'd know to squawk louder if it happened again!