One very common sight at long run events - men finishing the run with their tee-shirts completely red. Pretty amusing but very painful.
The fabric of your garment rubs the entire time you run and at some point the delicate skin breaks. Your blood mixes with the sweat on your tee and works its way downward. By the time you finish, it looks as if your chest has been crying tears of blood.
However, there is a simple way to prevent this agony and embarrassment.....vaseline. Just apply some before you begin running and you will be just fine.