Actually, my dive buddy was pretty experienced, and I think that was part of the problem...he was just complacent. You see the same thing with a lot of pilots who have several hundred hours under their belt. They have enough experience to be overconfident because they haven't had the shit scared out of them, and not enough to know what they don't know.

An amusing story from my last dive trip, where the DM had done his training a closed fist meant 0 whereas we'd all been trained that it meant 50. So for the first couple of dives before we'd asked why we were coming up early, he'd thought everyone had 50bar (a quarter-tank, for the non-divers) less than they had...