on some cars you can unplug the cam senosr or crank sensor before even attempton to start the car such as Astra's or Vectra's as they can run from either senosr, but if you unplug the crank senosr it takes alot longer for the engine to fire up as it doesn't detect crank/cam position on 1st engine revolution.
Also this can cause reduced power output (limp mode) but its not always the case.