ascent from 50'. This was years ago, when such things were included in "advanced" classes. It was pretty eye-opening, especially when I did my "flare" at 15', and found myself sinking back down again......it's hard to blow bubbles when you've got no bubbles to blow....
I've had one "sorta" OOA of my own, when my primary regulator failed while fighting a heavy current at 70' (I was on an older reg, and i literally "sucked" the exhaust valve through the port). Unable to get enough air, even when flowing the reg, I picked up my octopus and tried to use it. It flooded as well (it went in my mouth upside down, since that was the old NASDS setup. I later found out the exhaust valve had folded over when I jumped in).
Anyway, I was only able to get air by free flowing the regulator,and it was barely enough. I signalled to my "boat buddy", who I had just met, with a slash across my throat. He did the same sign back, shrugged his shoulders, and swam away.
I made most of my ascent with the free flowing reg, but at about 30' feet, I was completely out of air. I didn't have to drop my weights, but i did have to swim up. Boy, was I pissed.
On the other hand, I was diving on a rig once and saw a big shark come right up behind another "boat buddy", this one a woman who was a pretty good diver.
I laughed, which flooded my mask, then I placed my hand on my chest to say "Boy, that gave me a heart attack".
Before I knew it, she had pulled my regulator out and was shoving her primary in my mouth. She never saw the shark. I started laughing again, which caused me to choked, which in turn, almost caused a real emergency......it took a few minutes for me to sort things out with her. She over-reacted, but I was appreciative of her quick response.
Anyway, I have witnessed a couple of OOA's, but only had that one bad experience......it's one of the reason's I'm sometimes dive solo.