This is what happened to me last week:
A closer look at the failed hose, now conveniently laid open, revealed that the material is unusually thin. I remember noticing before that this hose was more flexible than the others, but I didn't make the mental connection… well. Anyway, this increased flexibility probably allowed the hose to kink near the 1st stage fitting, gradually weakening the already thin material on the inside without making it look suspicious on the outside, until, one day, ka-PFFFF…
So this hose turned out to have been a piece of garbage, and that's why it failed. However, we know that failures happen even with well-kept quality gear. They certainly did happen to me. The question is, would a similar failure be a problem during a dive? And the answer is no, but you have to stick to procedures.
On a single cylinder, being able to end a dive without undue risk (like a rapid ascent from 30 meters) absolutely requires that you stay close to your teammates and remain aware of their location so you can reach them in time. Remember, 30-ish seconds? Secondly, all team members must keep sufficient gas reserves. How much is sufficient? Here's how much: Every diver must be able to complete an ascent at the planned speed, without omitting any stops, while sharing gas with another diver, and with both divers breathing heavier than usual due to all the excitement going on. This amount of gas is called rock bottom or minimum gas, depending on who you listen to. Depending on your depth, this amount can be rather large.*
Surprisingly, major gas leaks become a much less hairy issue on more complex dives where people use multiple tanks for redundancy: You shut down the failed cylinder, switch to your backup, and abort the dive as a team, breathing your own gas. Sharing is still an option, with reserve calculations based on the principles outlined above, but it's not your first, last, and only recourse. Isn't that great?
Training and practice – responding to gas leaks and other contingencies correctly and reliably, with minimal effort and no additional complications (hesitation, trapped or tangled hoses, loss of buoyancy, wrong regulator…) takes more than just strapping on an extra tank or two. If you're interested in learning more, ask me about sidemount and technical diving courses. The skills you pick up in these courses, and the underlying mindset, will make you an even safer diver than you already are – even when you go back to ostensibly easy dives on a single tank.
Oh, and don't buy crappy hoses like I did. There, I warned you.
Safe diving, Tim
* To give you an idea: Based on an AL80/11 liter tank, an average diver uses about 1.5 bar per minute on the surface. (Some more, some less, so don't assume this applies to you!) 1.5 bar/min on the surface translates to 6 bar/min at 30 meters. Multiply that by two (two divers sharing) and then again by two (stress), and you're easily at over 20 bar per minute. You need maybe one minute at bottom depth to get sorted, and then you need to do an ascent with all required stops, ideally without running out of gas… If you want to learn more about this, feel free to ask.