I was teaching a scuba refresher group, and as they took their breaths to test their regs, I told them to look at the SPG to see if the needle was dropping. I told them if the valve was not fully open, it would drop and then recover, and if it was closed it would go to zero. As she looked at her SPG, a lady said, "So if the needle doesn't move, it's OK?" I replied, "It means that the valve is fully open."
So we went into the pool and started working on skills. Suddenly she panicked and indicated she was out of air. I shared air, and we went to the surface. I looked at her SPG, and she was indeed completely out of air.
How much air did you have in the tank when we started?" I asked.
"It was in the red," she said. "But the needle wasn't moving, so I figured it was OK."
Haha, while that is a funny story, it sure sounds like a case for stating how much gas you have as part of the pre-dive check before splashing.