The first two that came to mind were the worst two in connection with my pocket book.
The first: I had a lovely compact TUSA RS230 regulator that I recreationally dove for a couple years. A few years ago I was on one of our local dive charters and we had set up our gear, secured the tanks and were making our way to the dive site. It was a little choppy that day and well my nice little compact first stage was so compact with smooth edges that the bungee didn't hold it as secure as everyone else's and next thing we hear is a loud BOOM and a free flow. Sure enough, my tank had come unsecured, fallen, and knocked the first stage clean off the tank. Amazingly once the situation was contained the first stage reconnected, it still worked. I of course had the reg checked and cleared before continuing to dive it, but even after that about every 8-10 set-ups it would unseat and blow (a couple times while ON ME). So I drove to a shop in another city and had it looked at very carefully and the verdict was the first stage yoke nut was so minutely tilted (or bent) it was almost unnoticeable, but would allow at times for the O-ring to slip out and the first stage blow. I had to replace the yoke nut for a little over $100 in parts and labor, but moved completely to DIN regs shortly thereafter.
Hind-sight prevention: to have found another way to secure my tank than having the bungee simply snapped over my compact "edgeless" first stage. Not much for the bungee to grip.
The second: On the first dive of a Deep Diver Specialty Course I was teaching, my UK HID Light Cannon 100 went out at 90'. I had just changed the batteries and couldn't believe it was failing on me. So at closer inspection I see the pinched O-ring!!!! I completely flooded the light. It was pitch black at depth and I was left to guide my divers with my back-up "pen" light. Coincidentally I was diving off the same dive charter as I was when my reg bit the deck!
When I completed my dive and surfaced, I opened my light to see how bad the damage was, as I had never flooded a light. Who-boy that sucker was bubbling, fizzing, and frothing. There was nothing salvageable. It was $250 worth of toast.Hind-sight prevention: O-ring check.....dar!
In closing, which one hurt the most? Well the reg that blew on the Argon bottle this past weekend happened to be side mounted on my doubles upside down and pointed at my left cheek. I have a bruise the size of Montana and my rear end!
So I'd have to say that one "hurt" the most. [On a side educational equipment note, when using a dedicated reg/bottle for suit inflation purposes please have an Over Pressurization Valve (OPV) on the reg if it's not built into the reg already. Mine was built in, thank goodness, or my suit could have instantly inflated. Although my failure occurred on the surface during our safety checks AND my reg had a built in OPV, if it hadn't and the reg fully failed at depth, it could have shot me to the surface. Thanks to my training, I knew what appropriate equipment to be using instead of unknowingly attempting to mimic set-ups I've seen divers use.]
