One from my fundies - not nearly as hilarious as some of the posts here..
During basic 5 drills, I can't remember exactly how it happened, but I screwed up a couple of tries, and got lost in what state I was is. Ended up I was breathing off my backup and had my primary clipped off, but for some reason thought I was breathing from my primary. I felt a little tugging when I moved my head, so naturally thought that I had forgot to unclip my primary. So I unclipped it. (The tugging was actually do to my hose routing not being ideal, and the backup reg hose being shorter than it should have been).
So now I'm breathing off my backup, thinking its my primary, and my primary is actually floating around behind my head somewhere while I complete my basic 5. The instructor obviously sees this, and tells me to do an S-drill with my buddy OOG. She didn't notice the sorry state of my breathing gear, and signals OOG and drops her reg. I remove my backup from my mouth, wondering what the hell its getting stuck on! Needless to say the instructor intervened at that time and got me to re-do my Basic 5 and S-drill
Now onto a better story, relayed from another diver who had just completed Tech-1 on the same day. The students had completed most of their dives, and were on the last pleasure dive for the course. Let's call them Bob and Sam.
Weather was rough as guts, wind started to pick up, and 1-2m swells started coming in on a boat full of OW, AOW and Tech1 students. Bob, Sam and Instructor trying to manouver with their rigs around OW students hurling over the side and stern of boat, eventually get into the water. Bob was on deco duty, Sam was on SMB deploment.
On ascent, they hit their first deep stop, Bob resets his bottom timer for the stop. Screen goes dead. No drama, Sam takes over deco and they continue their ascent. Bob's now on SMB deployment, but his SMB has gone AWOL. No drama, Sam pulls his out and hands it across. Time to switch gases. Bob turns on his stage, sees the guage start to move, switches regs and goes back to monitoring deco time. Couple of minutes later his stage is OOG! (Most likely the valve may have been knocked open on the busy boat, and gas escaped through the current on descent). He signals OOG and the three finish their dive sharing deco gas.