Keyless entry when combined with keyless ignition.
For such a gimmicky thing, it soon becomes 2nd nature, and annoying when you jump in a car without it
The thing I've done with keyless entry cars, is when I park and walk away, but then because I have OCD or some other syndrome, I always have to go back and double check I've locked up.
And of course when I check the door it's open!
A guy I work with pulled in to our works car park, got out to chat to us, pressed the remote and locked his car as we walked off to do our shift.
In the early hours of the morning, as we'd finished our shift, we were walking out to the car park when he noticed his lights were on. Thinking that someone was about to steal it we ran over to his car, which had the engine running. His car was still locked, nobody was in it, the engine was running and the auto lights had come on as it was dark.
That car, a 530d, had been sat idling for 11.5 hours.
It had used less that a quarter of a tank.
The only saving grace was, it was minus five that night so he didn't have to defrost his car and it was lovely and warm inside.