Then you catch up to her or she turns around, only to reveal that she is a man, or incredibly ugly woman. The blood that had engorged your penis is now forced back into your system by your penis retracting into you body at light speed. This wave of high pressure blood, driven backwards through your system blows a hole in your heart and you drop dead on the spot.
It's not so much that she is ugly, it's that she is so incredibly sexy and arousing from behind, then the shock. Imagine a wiener on the grill fully cooked and hot, then dropped into a vat of dry ice.
I'll miss that horny bastard, the funny thing is, he's fucked uglier girls, it was just the shock.