Right after we just started diving, swimming too hard against a current in Puget Sound, my wife was over breathing and sucked her tank down as we were heading in to shallow water for a safety stop. It was my first dive in a dry suit, we handled the out of air situation fine, but it was a challenge to maintain buoyancy control using an integrated air source.
Another time, while practicing in a lake, my smooth new steel tank slid out of a brand new BC strap, and pulled the quick connect loose from my integrated air-source. I passed my primary to my wife ( practicing ), exhaled hard to clear my safe second, and tried to suck in some air, and there was nothing there. Suprise, suprise, suprise! It lead to some very entertaining hand signals as I tried to convince my wife to give me my regulator back!
We're both much safer and smarter now!
I also remember an incident in Cozumel, with a young lady on her first dive. At 80' on Santa Rosa Wall, she spit out her regulator because she was convinced she wasn't getting enough air! Yikes. She was actually with another group, but our DiveMaster and I arrived at about the same time, and he stuffed her reg back in her mouth. DM then "escorted" her to the surface.
Boydski