I was in Cozumel a few years back on a trip with a group from a local dive shop.
We were on a surface interval, anchored over a shallow reef, and I decided to make a quick bounce dive using my Spare Air.
I hopped in with nothing but my wetsuit, fins, mask, and Spare Air, and found myself fighting to stay down, and rapidly depleted the small amount of gas in the cylinder. With no warning, I was suddenly drawing no gas at all, and I was down around 25 feet or so, and I was winded from trying to overcome my positive bouyancy.
I rapidly ascended, blowing bubbles the entire time and thinking how stupid I was for getting myself in that predicament. I got to the surface and took several large breaths of air and thanked my lucky stars for getting me out of that one.