Curreny: coins flung from speeding vehicles hurt. Pennies not as bad as quarters, but most of the as$h0les who do that kind of crap are cheap, poor bastards, so they can't afford to jettison the higher-valued coinage without fearing the forfeit of their next beer.
Bottles: thankfully, I haven't taken a direct hit with one...just a couple meant to disintegrate upon impact in my immediate path so as to cause a flat. Dumb as$h0le obviously never heard of kevlar-belted tires.
Spit: need I elaborate?
Slap on the ass: not me, but a guy in front of me was the unfortunate recipient of a slap on the backside by a group of roudy kids passing in a convertible.