Yup... Several years ago, I was diving in Utila. I was staying at a nice resort that had that fancy "Valet Diving" crap. Several days into the week, on our second dive of the day, I was putzing around in 90' of water, with my brand new extremely easy-breathing Apeks reg. At one point about 20 minutes into the dive, I took a breath and found it a bit "tight", then took another and got nothing! A quick glance at my (floppy) gauge confirmed what was already apparent... zero gas!
Now bear in mind, that I am of an age that we trained doing actual free swimming ascents, and for many years, I used to do them pretty matter-of-factly on many dives. None-the-less, since this was the second dive of the forth day of diving, I figured I would almost certainly bend myself like a pretzel, so I wandered over to Mrs. Stoo who was looking for little stuff about 25 feet away. I borrowed her octopus and we went about packing up our camera stuff and prepared to ascend. At that point our DM came over, asked what the problem was, and burst out laughing when I indicated that I had run out of air! ;-)
Anyway, we headed up, shot a bag and completed a safety stop before returning to the boat for a fresh tank.
After the dive, it became obvious that the "valet" who was to have switched my tank over between dives had neglected to. (The same DM who laughed at me!). I had begun the dive with about 900 PSI and of course I am an idiot for failing to check my gauge before falling out of the boat.
The poor DM (who I had known from a previous trip and I consider a friend to this day) was horrified at our mutual screw-up so we drowned our sorrows with a "little" tequila at Tranqs that night. What else was I to do, having survived a near-death experience? He and I agreed that it was just as well that this little oversight occurred on my kit, and not a less-freakable guest's!