There´s a thread on here right now about how the kind of diver you describe did that.Blox:Given the existing structure of courses, standards, etc., I think it is much more preferable to permit a new diver with less than 10 dives (OW + AOW) to go straight on to the Deep Dive Spec and get in some real world deep dives under instructor supervision, and gain some more knowledge and experience before venturing out on his own, rather than the same guy coming out of AOW with the "I'm certified = qualified for deep dives" perception, and doing deep dives with just one single prior deep dive under his belt (from AOW).
He went down with a pony (that it seemed he´d never used before), two other students and an instructor. One of those students was a "trainwreck" (the OPs term, not mine) so the instructor was paying attention to this @ 90ft. The OP then had a freeflow and (accidently) went to his AAS, he thought he was breathing his pony. When he alerted the instructor to the situation he proceeded to shut off his tank (which he was breathing from :11: ). Panic ensued and he bolted to the surface. What happend to "trainwreck" during this is unclear but it seems everyone survived the dive. The OPs "lesson learned" was to make sure he could find his pony-reg on the next dive :11: :shakehead
That poster had at least a few dives in before diving with "instructor supervision", I´d hate to see someone with 9 lifetime dives try to survive that sort of instruction...With a few dives you have, at least a reasonable chance, to have some sort of experience that will allow you to evaluate whether a dive-plan is "sane" or not...
I think you have an unreasonable faith in instructors...