Nick's rock reminded me of the time a bumble bee hit me square in the visor. Now a bumble bee is big, but not that heavy right? No matter at 50 mph. I was cruising along and whammo! Big, fat bee hit me square in the visor, damn near knocked me off the bike and pushed my head back about a foot. It broke the visor plastic (cracked). I can't imagine if that thing had hit me in the face and stung me to boot.
A friend of mine wore a 3/4 helmet with sunglasses and got hit near the eye by a bee while he was doing just about 50. The thing left a mark on his face like he'd just been decked by Mike Tyson.