Given JBZ's story, this seems apropos:
I work on the Penn campus. Last winter, I talked for a little while with one of the campus police officers, who had questions about the building, which doors were normally left unlocked, etc etc. Bland stuff. Anyway, he was an easygoing guy, joked around a bit. Seemed pretty cool. As I'm leaving work a couple hours later, I see that he and three other campus police officers are standing and chatting in the plaza outside. I walk through the (ice-covered) plaza on my way out, and I give him the little head nod that guys give each other when they want to acknowledge the other guy but you both know you don't really have anything to talk about. RIGHT as I give him The Nod, my right foot goes completely out from under me; I go completely horizontal in mid-air. Perfectly parallel with the ground, something straight out of a cartoon. I have enough presence of mind to half-toss my bag off to the right so that I don't land on it and break my camera, phone, sunglasses, etc, and land flat on my back with a thud. Wind completely knocked out of me. I look up and all four guys are staring at me with that face that says "Holy shit, that was hilarious, but if this guy's seriously hurt and I'm laughing, I'm be an asshole." So I sat up and said "Perfect 10 on the landing!" and they all started laughing hysterically. I picked up my bag, grinned sheepishly, and walked away VERY CAREFULLY. No head nods on the way out.