Twice - once when I was the donor and once when I was OOA. I was the donor when a diver on a group trip I was leading ran out of air at about 80' and was separated from his buddy. He gave me a non-standard hand signal that I could only interpret as "I've reached 700 PSI and need to surface." I signaled him to buddy up and ascend, then located his buddy and swam after the buddy to get the buddy team back together. I looked over my shoulder during the swim and saw the guy headed for the surface, reg in his mouth, looking more or less under control. He was doing a decent emergency swimming ascent. I took off after him to chase him down, grabbed a fin, crawled up his body, offered my primary, which he took, and switched to my AIR2. By the time I got us stabilized, we were at six feet. I thought about descending to 15' to hang for a few minutes (I had plenty of air) and then decided that we were shallow enough to bring on all kinds of bad things and that I would rather have them happen at the surface. We surfaced, I got him to orally inflate, then towed him back to the boat. He was pretty much out of breath at this point. Luckily, we both came out of it OK. I learned a couple of things on that one and would be unlikely to try to chase somebody down again. I will also make sure to dump all air from my BC before sharing air, to avoid a runaway ascent.
I was OOA myself recently diving the William A. Young in the Straits of Mackinac. My reg froze up and went into a free flow about five minutes into the dive in 42 degree water at 101'. I grabbed my buddy, showed him the reg was free flowing, and headed for the line, still breathing off the free-flowing reg. We started our ascent and part-way up, when I reached about 600 PSI, I signalled low on air and asked to share. He donated the reg on his pony, we made a good ascent and a good safety stop. Having a good buddy sure helps.
