It happens - things like that are like a right-of-passage.

You know those old westerns where the revolvers don't run out of bullets?
I recently took quite a few pictures (via three 120 backs) with the auto-exposure lock accidentally off; I'd not set it, but it had been bumped when pulled from the bag. It was in low light, so I couldn't see the counter, and it never seemed to get to the end of the roll.

Fortunately I realized the issue before unloading, so I can take the other 9 shots on each roll. Lost a lot of good pictures, though.