I was leading a group on the coast in April a few years back with my half helmet and no windshield on my Fatboy. I saw something in the air but I was sure it would be gone by the time I got there. It was not and I don't have a clue as to what it was, but the guys behind me said my head snapped around like a took a left hook from George Foreman. I thought it was going to knock me off the bike for a few seconds until I got my bearings again. It left a mark on my right cheek for days---still don't know what it was.
BTW---I have been hit twice by a golf ball. The man who stays on his bike after that wins hands down.