It happened on my checkout dive, first time out of the pool. My octopus got caught behind my tank, and my "buddy" swam off to play with his friend. I actually turned to the surface, and then heard a voice in my head say "Don't do that, you'll die." Then I saw my "buddy" swam to him and ripped the regulator out of his mouth.
As a former PADI instructor, I would have to either call BS or say you had an impossibly lousy instructor and dive master during your "checkout dive".