I've been using chainsaws for many years, have probably owned 5 or 6. These days it's a Stihl Woodboss MS270, 24" bar. Every year I pick up mostly wind-felled oak on country roads, haul it home, cut it into stove-size lengths, then rent a splitter for a day and stockpile a year's or more worth of firewood. I used chainsaws extensively in the '60s and '70s, cutting up redwood (from the beaches or wind-fallen trees in the woods) into bolts, and which I then split into shakes. Point is, I've had a lot of chainsaw experience.
The other day I was sawing through a piece of wood on the woodpile and as I finished the cut, the blade hit a log below it and snapped back towards my face. It sent a chill of adrenaline that I felt somehow in my ears.
I was wearing my Husqvarna helmet, which combines skull protection, ear guards, and a metal mesh face mask. (I've only been using the helmet the last few years, prompted by a log rolling down the hill and knocking me down — I should have had one of these helmets all along.)
This time the blade didn't reach my face, but if it had, the mask would've stopped it from carving up flesh.
I urge you chain saw users: get one of these; $50 or so. Play it safe, please. The more hours you've operated chainsaws, the more the chance of a freak accident. Experience doesn't make you invulnerable.
P.S. I always sharpen the teeth after use, so it's ready when I next pick it up.