Blackwood
Contributor
I have never run out of air/gas, and I would regard it as a very serious matter if I ever did. I had an occasion a while back when on a recreational dive one of the group did run out of air. I put him on my octopus, called the dive, and sent up my DSMB. While I was starting to reel in the line (we were at around 50ft) one of the other divers also ran out of air and grabbed my reg from my mouth. This second diver was moderately panicked so I left him on my primary second stage, ensuring he had a good grip on my harness, and started to buddy breathe with the other guy who was on my octopus. Doing that whilst reeling in the line was quite tricky, plus controlling the buoyancy of the slightly panicked diver who was all for bolting to the surface. That was an interesting dive, moderately stressful, mainly in hoping that I wouldn't run out of air (with two people hanging off me I couldn't see or reach my SPG).
If that isn't stressful, you're inhuman.
If it happens (which yes, it a serious matter) in the confines of an otherwise well oiled team, not so much.