Everyone has their own opinion on this situation but I have to tell my experience this summer on Thomson flight from Turkey.
We had just been pushed back from terminal at about 1am and we began to taxi (very fast) down to the runway. As we were going so fast we all looked forward and I saw that the flight deck door was wide open. I don't know who would be to fault over this but don't the cabin crew sit at the front right next to the flight deck door? They should have seen it was open and shut it.
Before we took off it was closed but I didn't see who actually closed it.
The aircraft was a Thomsonfly 767.