I just remembered another OOA experience of mine, one that might well explain how it can happen--although I am at least partially to blame.
It was a refresher course in a swimming pool, with a group of 6 divers. We finished the equipment setup (including checking SPGs, of course) and moved on to predive safety check. When it came time to check air, I gave the standard warning about making sure that the tank valve was open by looking at the SPG while taking several breaths to make sure the needle doesn't drop.
A woman said, "So if the needle doesn't drop at all, you are OK?"
I replied, "It means your valve is open."
So we got into the pool and started working on skills. We were no more than 5 minutes into the session when she got a panicked look on her face and signalled OOA. I was fortunately right next to her and was able to donate. At the surface, I looked at her SPG and saw it was dead empty. "What was your SPG reading when you checked it?" I asked.
"It was down in the red," she said, "but the needle wasn't dropping, so I figured I was OK."
After that, I added a comment in my instruction about the amount of air that should be in the tank and why it is not a good idea to start a dive with an empty tank.