First I want to reiterate my early comments with respect to an instructor telling a student to abort a takeoff if a cabin door opens “because the door could fall off and hit the aircraft”. I think this is utter rubbish and needs to get stamped out now.
LOL.
The logic was: If it opens, especially if it opens after you already checked it was properly shut, something might have broken that could lead to the door coming off in flight.
It's not gonna cost anything to stop and investigate - and it's not rubbish, as can be seen in the case of G-ASOS last year, where the baggage door wrapped itself around the tailplane and created control difficulty.
I'd rather abort than be a test pilot.
Obviously, the situation is different if you don't have enough runway to stop... but I thought there was enough common sense on this forum that I didn't have to point that out.