As for an errant kick knocking my mask off, I've only had to deal with this once, as a brand new diver. I was lucky in that the mask was close enough to easily retrieve. That said, when my dad and I dive together we spend a good 1/4 of the dive antagonizing each other- removing fins, grabbing masks, etc. We do it safely, so we're always around if the other needs help or (eek) is about to hit coral, but I've grown accustomed (as accustomed as one can grow) to swimming around in salt water with eyes open. After reading this thread though, I may invest in another mask this week. Better safe than sorry, no?
-Ben