A couple of weeks ago I was doing my routine checks and maintenance on my wife's W202. Its equipped with the South African spec electronic immobiliser and antitheft, which has an interesting feature - a preset time after starting the engine (probably a minute) the doors automatically lock. So I started the engine, and casually closed the driver's door (as I would with proper '70's benzes), and you guessed it - the car locked itself, with the engine running, and nearly a full tank of petrol. hmmm. It did make me stand and reflect for a while. And no, I don't have spare keys for the car (real slack of me - and it came to bite me).
I eventually got lucky and found that if I pulled a certain fuse, the security system released. Whew.