I have had two. The first one was a diver who was "experienced", but hadn't dove in quite a while. We were at a "check out" type site and it was only 25 minutes into the dive at about 45'. I had already checked her air once about 15 minutes or so into the dive and she was at about a half tank. I was actually just getting ready to check her again when she gave me the OOA. Her husband was right beside her, but she swam to me (thankfully) and I donated my prmary and we swam slowly back to the boat rather than straight up. She got on the boat and was happy that everything came back to her like it did. I am still not sure how she went from a half tank to no air in that short of time, but she did. I was paying more attention to their son, who was 15 for most of the dive, but knew she was breathing heavy.
The second one was another woman who had been on a few dives that week already. She gave me the half tank signal, but didnt give me the low on air signal, just straight to OOA. Another AOW certified diver. She swam right for me agian, rather than her husband and we did the drill and surfaced from 40'. Both people commented on how calm they felt during the situation. I was happy they were calm also....
The moral for me is though, it does happen. Luckily I was close enough to help. Either "buddy" could have probably gone either way with helping, or possibly not reacting right and causing a panick.