You would most likely be unable to open the door during the take-off roll. It is a plug type door, which means it has to move in before it can move out. The aircraft is slightly pressurised during the take-off roll, so the chances someone could get it to move in against the pressurisation is slim, if not impossible.
In my experience, a warning light after pressurisation usually means that the handle has moved slightly up - either by vibration or someone playing with it.