Billy Horschel left a fan with a Hor-rible wound while out on the golf course Thursday -- after he accidentally smacked her right in the leg with an errant tee shot!!
It all went down at the RBC Heritage in South Carolina ... as the PGA Tour star was trying to pump one of his drives deep down the fairway on the par 4, 10th hole.
Horschel gave his ball a ride ... but, unfortunately for him, it leaked right -- and came crashing down right on a woman's shin.