Well...a couple. I actually had a student breathe a steel 72 dry in a pool training session (basic open water class). I am still wondering how he actually managed that. I always pulled the student tanks out of the shop racks and checked pressure on each one before class, topped off any that were even remotely low, and then before I let them in the pool they had to tell me how much air was in their tank before we went into the water. We were about a half hour into the pool session and to that point in my teaching I usually looked at my gauges, then pointed to each student in turn for them to check with the rule being if they had more than 1000 psi they were to give me an okay sign back on the air check which I did at 15 minute intervals. How this guy managed to go dry so quickly was and still is a mystery to me. He didn't appear stressed and wasn't breathing in a way to cause me alarm, but something happened. The deep end at this pool is 15 feet and I gave him my primary and went to my secondary. Needless to say from that point onward I started looking at student gauges at about 15 minute intervals.
The only "save" I really made came as a total surprise and really isn't another diver. In a previous life I was a police officer in the pacific northwest and was a member of the dive team. We got called out for a boat floating upside down about 200 yards offshore. Usually these things are someone trying to collect insurance money by scuttling their own property, especially since the weather had been calm for days. It was about a 25' cabin cruiser and my buddy and I went under the boat and swam up into the cabin to find a pair of legs dangling down, kicking. I don't know who was more surprised...him or us for actually finding someone in flipped vessel. I surfaced in the air pocket, which scared the living crap out of him by the way, and gave him the half minute version of how to breathe through a regulator, and then we took him out of the boat. The person told us that he had decided to sleep aboard and woke up with "everything upside down" (sounded fishy to me...but who knows, the Coast Guard had to deal with the whys after that).
