I did the support diver work on a deep exped one time (172m to seabed) and I was given two divers whose job it was to look after the deco station and stage a couple of extra bottles. This involved diving to 12m max and as one of them was a fully certified PADI instructor dive leader rescue and whatever other tickets he'd acquired, I had no worries about him. That was until he went over the side in full deep water rig, found himself over-weighted and descended into total, unreasoned panic. He was sinking and would undoubtedly have died but for my deck-hand gaffing him with a boat hook. We pulled him alongside, threw a loop around his manifold and left him dangling from a cleat until he calmed down. It was later that I learned he'd got all his tickets with the minimum number of dives and actually couldn't dive at all.I suppose that there is nothing to stop a reckless and over confident individual obtaining their Open Water certification and, with eight or nine dives in their logbook, buying a tank and a regulator and going straight out and getting themselves into a situation which they can't handle.....
We had to go for fuel soon after and he got off the boat with his mate never to return.
When we got back to the dive site we discovered that the girlfriend of one of the bottom divers (now 150m below the surface on the wreck) had volunteered to do support so she went over the wall but without checking what was in the cylinder she'd picked up, which turned out to be a trimix that was unbreathable at the surface. She went hypoxic, lost consciousness very rapidly and became the second casualty to be gaffed that day by my deck-hand. This one was more serious and had to be resuscitated then rushed to hospital where, thankfully, she made a full recovery. Those two very nearly did pay and die immediately, and neither went more than a few feet below the surface.
I did a quick Google on it so I could supply a link but only found discussions on various fora but one astonishing finding is that the dope who panicked and had to be pulled crying like a baby out of the sea later claimed to have actually dived the wreck!