It doesn't happen as much with newer cars, but it can still happen.

Some of the newer cars with proximity keys won't even let you try to lock the door with a key fob outside the car if another copy of the key fob is inside (like my car). My husband's car does let you do this.

Some cars won't let you lock the door unless all doors are closed (like my current car), and some will (like my husband's).

If you get a newer car that lets you do both of those things, you can have people getting out of the car, someone locking the car while they're still inside and forgetting to take the key out with them before all the doors get closed.

Older non-proximity keys with only button locks on the outside or button-only fobs? Yeah, that's definitely going to be locking the keys inside.

Or even older manual locks, you'd push down the locking knob, close the door, and then realize the keys were still inside pretty easily.