I don't think I've ever had anything fail on me diving, but I've been with people who have run out of air (my buddy was constantly using all his up), so having to breathe off someone else's tank isn't uncommon.
As I mentioned in my sibling comment, I did have a scary time on the Blue Hole. I think my other most nervous dives were:
- Pacific dive in Costa Rica in rough seas and surge. We suddenly had visibility drop to near zero when we hit the outflow current of a river. Definitely a lesson in how quick conditions can change.
- Cavern diving in a cenote in Mexico. Nothing weird happened, but we went kinda far in, and I get nervous in overhead environments.
Running out of gas or having to breathe off of someone else's tank is uncommon. Gas planning and monitoring is a fundamental skill that every diver needs to master just to get a basic open water certification. If your buddies aren't able to do this reliably then they need remedial training.
It's good to practice gas sharing as a contingency in case of equipment failure but actually running out is not acceptable.
Yeah this is no laughing matter, i would refuse to dive with someone who lacks the basic skills to monitor their air responsibly.