The most painful shot I ever took came in from behind my mask and hit me in the back of my jawbone under my ear. I had an old mask on and was fogged up and didn't see the guy hiding 5-7 feet away
I have a flex 7 ize mask now, and the whole bottom of it is designed to be flexy so the paint won't break when it hits. Its flexy enough to slam you in the face every time you get shot there, but at least the paint hardly ever breaks.