1. (i was drunk-ish :)). Demanded (And got) a massage from a guy who didn't know what else to do.
2. As a kid, went and sat on a random man's lap mistaking him to be my dad. I wasn't even that young, and I was also a chubby kid. This "uncle" actually never visited us after that.
3. (i was drunk. very. :)) Whirled around showing my middle finger to all and sundry at a bar.
4. Meeting a bunch of people for the first time. One of them goes "she looks like xyz, doesn't she?" to the rest. XYZ being the name of a famous, very beautiful actress. Instead of shutting up and smiling gracefully, or saying "I beg your pardon" (because that would've been the right thing to say in my case) , I go "Oh god, i'm sure she's turning in her grave right now". The group looks pretty disturbed and confused. Turns out later, the person didn't say xyz, but a name that sounds like xyz, who was apparently some common friend of all of them, and yeah, very much alive.
Never met these people again either.
5. In my first year here in the US, went and wished "A very Merry Christmas" to a very senior staid professor..who was jewish. Heh.
OK NOW, YOU GO!