Once when playing 3B in high school, I was talking to the SS (who, by the way is my Marlins/Nats buddy down here) when I heard the crack of the bat - they were still wood back then. I turned just in time for the ball to smack me on the bridge of my nose. I stumbled around, grabbed the ball, and heaved the mother to RF, continued stumbling, and crashed on the bench. I wore the stitch marks from the ball for a long time, and still have a scar from that one.
I was playing in an aluminum bat league (18 years and up - I was 34) back in 2005. My first game from out behind the plate I played 3B and the leadoff hitter hits the second pitch of the game - a screamer that took one hop and hit me in the forehead and knocked me unconscious - and I was paying attention and had my glove down in the fielding position. Our 1B said it looked like I "had been shot" because my body went limp and I ended up landing flat on my back. After coming to, I found out that the coach had replaced me and I begged to stay in. He let me play another three innings until I chased a foul pop-up to just beyond the dugout and went over the fence to catch it and it landed just out of play. In frustration I slammed the top of the fence with my open hand and cut myself wide open at which point I had to be removed for the league's "open wound" policy.
Talk about a bad day. Plus I struck out in my only plate appearance.