My parents had a Mark 2 Ford Escort. My mum came home in someone else’s car once, as it looked similar and the key worked in it.

Mazda 323’s used to open each other too, and also Ford Lazers. These problems don’t seem to happen anymore, but at some cost.

eh... my car before that, a 90s Japanese car, had plain old dumb keys. It was stolen twice (and found abandoned twice) by joy riders who easily stole it.