Long time member of BSAC here. The classic example I use when describing this to students is torch batteries. If you're at home and you notice that your dive light batteries are flat, no biggie, just swap them out. If you notice while on the boat that's a bit more annoying, but the chances are someone will have a spare you can borrow, and if they don't, you will have to miss the dive, which is unfortunate but you're not in any danger. If you're in the water, you can still call the dive (abort and return to the boat). But if you're at 50m and entered a wreck and your buddy has just swum round a corner taking their light with them and you switch yours on and nothing happens, then you have the rest of your life to try and find the way out.<p>Same, seemingly trivial, failure, but very different consequences depending on at which point of the dive you notice it.<p>By the way I mainly dive with GUE now who mandate a primary light and two backups as part of standard kit!