I had a tournament this Monday and Tuesday and, on my way out the door on Monday morning, received a mild concussion before playing. It was stupid, looking down to check an iron while walking swiftly out the door and running straight into a wall, but it's surprising how much that effects you. I probably only went through my pre-shot routine for about half of the shots I can remember just because I would forget to do it. I was late to my tee time for the first time in about five years of tournament play, and I played like poo. On the plus side, it was easy to forget the bad shots and move on!
On a more serious note, I have found out that this is a very poor decision from a health standpoint. I didn't know I had a concussion at the time (I was dazed and suspected it, but didn't schedule a doctor's appointment) and played on. The best thing to do, I've been told, is to just wait for it to heal. All-day headaches are no fun when you're trying to play your best.
Anyways, that was just my little PSA from a recent personal experience.