BBs leave marks once in a while, but nothing bad. Of course, unless you are stupid and aren't wearing eye protection, which for me on my ESS Turbos covers my ears as well. Field injuries happen more than AEG fire injuries from my experience.
My worst BB scare was in a op a few years back in NY. I was crawling along side a bank on a river, and I was point. We were coming up to an old abandoned concrete tube of some sort, and I was moving up to it to be the patsy and look for signs of life. There was, I spooked the guy from about 20 ft, and he turned and started blind firing right as I was moving from the tree I was behind to another along the bank. I felt a shot hit my lip, and it was pretty painful. I felt something moving around in my mouth, I was like shit, I just broke my tooth. So I spit it out, covered in blood, and its another BB. The dude blind fired two straight into my mouth. Suffice to say, though it hurts getting hit on the lip, I was relieved to be spitting out plastic instead of teeth.
At Red Dagger I got "injured" trying to get into the halftrack. I flipped into it, since I am acrobatically and horizontally challanged, and landed on the floor with my fingers between my AEG and the metal grating. Didn't even notice I cut my finger, but it has left an interesting scar since then. Thats all I got lol.