Slight thread necromancy here, came across it looking for something else, but I had no idea an upside down reg would cause this problem at any stage during training. If it was mentioned, it must have been a couple of seconds tops and instantly forgettable in how it was presented.
I first learned how it could happen three weeks ago. Of all things I was watching the special features on the making of The Abyss DVD! The part when Ed Harris drops off the wall in particular, he was holding his breath being towed sideways (to simulate falling without a need to equalise) then gave the out of air signal. The rescue diver was, unknown at that stage, tangled in wire so a camera operator rushed in to give Harris his reg and ended up doing it upside down. Harris tried to breathe normally and got a full on wet breath but someone else had arrived by then, recognised and corrected the situation. Kinda shook him up.
Does make me wonder how many other new divers don't know this.