There was an episode of Car Talk where a caller described this exact situation. In that case, for the model in question, there were only so many varieties of keys made, so you could use your key on some other cars.
I had a friend in college. On his car, any key[1] would turn the driver's side door lock. Not the ignition, though. You could break into his car, but you couldn't drive it off.
[1] For the nitpickers: No, not a skeleton key. Some would probably be too fat to fit in the lock. And so on. But virtually any car key, and maybe even a house key. It was a Chevy, IIRC, but I opened it with a VW key.