In high school, I had a '64 GTO. My buddy has '66 396 SS Chevelle. His key would unlock my doors but wouldn't turn the ignition. My key worked in everything on his car. Guess who had the most fun hiding the other's car? There were a few other GMs that we could unlock, but nothing worth messing with. It's not as uncommon for keys to fit other cars as one might think.
And for the record, his Chevelle never beat my Goat at the local Shuffletown drag strip!