This is legitimately terrifying. Pirates shortstop Jordy Mercer hit a line drive right back at Marlins pitcher Dan Jennings and hit him square in the head. He immediately went down to the ground and then scrambled around on the mound before getting up, freezing, staring blankly into space, and then staggering around before teammates and trainers got to him.
In the replay at the end of the video, you can really see how flush the ball hit him on the side of the head. I can't remember seeing a player react like that, it looked more like a machine malfunctioning than an injured person. Jennings was awake and on one knee for most of the time and acknowledged the crowd as he was carted off, which is more than can be expected after watching that.