Only time I have HAD to bailout to OC for real (i.e. not for training purposes, etc.) was a few years ago when I was demo diving a Liberty SM unit. The owner of the unit who was giving me the demo, and with me on the dive, had apparently already run 1 too many demos that day without having refilled the tiny onboard O2 supply. I'd say this was his fault, but let's just say it was my fault for trusting him and not checking myself before the dive. So what happened was we did a 20 or 30 minute dive to 40 or 50 feet depth, and as we ascended the PPO2 kept dropping even as I kept hitting the O2 MAV. When the PPO2 hit 0.17 and I still had not figured out what the problem was, I bailed out to OC. Since at this point we were at 10 feet, I could have just done a CESA at that point. If I had realized what the real issue was, I could have just done a dil flush to bring it back up to 0.21 or more, but since I was in doubt, I bailed out.