I am really struck on this one that people are reporting relatively few at depth OOA situations--as others have said on this board that is I think a testament to the quality of modern equipment and quite heartening.
Ok, so I report one of about 3 of 4 cases I was involved with...
The first one it was ME!
Despite I was using a twin tank, I had just one reg (I was a young student, no money). My girlfriend (now my wife) also had a twin tank, but again just one reg. We were on holiday at the Elba Island , in July 1979.
I made a typical, stupid novice error: I did not open entirely the valve, just one turn or one and and half, no more.
We were not very deep, just 30m (our typical dives, at the times, were around 50m), on a vertical wall covered by paramuricee and with small caverns going inside the wall, where lobster were making their "cree cree" noise.
Of course we wanted to get them for lunch. At the time, reverse kicking had not been invented yet (or, in any case, we did not know this technique), so the method was that I did enter in one of those small cavern, and after a while my girlfriend did pull me out from the end of my long freediving fins. I entered in a very narrow tunnel, captured the lobster, and when pulling me out, the valve did hit several times against the rock ceiling. So it actually closed! As I exited and faced my girlfriend I signaled her that I had no air, so we started buddy breathing and ascending. It was not easy, but we made it up to the top of the vertical wall, where there was a plateau, at 10m, where we stayed for calming our breathing. Just at that point I had the lucidity of thinking to the valve, reached it and opened, and I had air again.
We continued the dive at low depth, as the bag was full of lobsters already...
As soon as we were back to the diving shop, we did purchase other two regs, one each, for avoiding this problem again.
In the following years we did use the second reg several times for giving air to others, but none of us was OOG again. We never had the occasion to buddy-breath from a single reg again...