I've had gas in the face a couple of times that I can remember: at least once or twice with .22 rimfires where the rim's been pierced - so long ago I don't remember the details but I suspect that a combination of old rifle and old ammo was at fault. I also remember an old hammer SxS shotgun which pierced a primer and let me feel it, again a long time ago.
I've had the odd couple of case-head separations in Lee Enfields, though not for a good while since I found a way to avoid them. I'd have to say I didn't even notice until I went to chamber the next round and couldn't. I think that the rimmed case-head and the case pressing against the chamber wall confined most of the gas, and the L-E's venting did the rest. I've also had the odd pierced primer in one or two rifles, but again not noticed until I looked at the fired round.
What I haven't had though is a case-head actually failing. I suspect that this is where the gas-handling really comes into play. A bit of gas escaping into the chamber wall from a case separation, or pushing back against the firing pin through a pierced primer, is rather a different thing from a case head actually letting go, allowing gas to vent through, say, an extractor cut. I also suspect that this was a great deal more common a century or so ago, when the consistency of early propellant and brass was less. Having said that, a prudent designer still has to consider the possibility of these and of a whole range of what are sometimes euphemistically called "human factors" - from my time analysing various failures it is clear to me that there's no such thing as truly idiotproof. Given the choice I'd rather have a design that can cope with failure of whatever sort,but to be honest it is not something I spend a lot of time thinking about.
