I personally have been out of air twice in my career. The first time I was a newbie & still overwhelmed by keeping up with everything. In the murky quarry we were diving, I lost my buddies (we were a group of 3) in a group passing by. It took me a little while to figure I was with the wrong group. I found them relatively quickly, but was critically low on air. We did our safety stop & I ran out of air just as I broke the surface. My buddies were aware of my problem & had their alternate air sources ready for me if I needed it. The second time I was boat diving in the Bahamas. I was buddied with a mom & 2 daughters in a much larger group. We had to navigate a ways to the reef from the boat & one in the group decided to leave a marker for everyone to follow back to the boat. She prematurely pulled the marker up thinking everyone else had gone back to the boat. My group was getting ready to go back & low on air, but not yet critical. We searched for a while to find the marker.... no luck. One of the daughters & I got caught in a strong current at the edge of the reef. The mother & other daughter were able to get out of it in time, but the daughter I was with & I could not get out of it. By this time we were all starting to get critical on air. The daughter I was with & I tried to swim perpendicular to the current to get back to the boat, no luck there either (the current was running parallel to the boat, but a ways off). I made the decision to come up shallower to about 10 ft, knowing chances were that we would run out of air. After a little bit of fighting the current my buddy & I ran out of air at about the same time. Still caught in the current, we signaled the boat for assistance. They sent out a dinghy to pick us up. The major lesson learned there, become better at navigation (which I have). Another lesson learned,... call for help from the boat a little sooner. We were both OK, but that dive could have been handled a bit better.