On my friends 18th at a club in the town centre of where I live, I was chatting to this really hot girl, another friend had invited her along. Anyway we were getting pretty deep into, I got her number etc, then I went for a drink and there is my friend (Who invited her) giving me a fairly evil look, and then the girl I was chatting to, who was 23, said "Are you 16? Sammie said you are...."
Damnit. That's the biggest epic fail I have, except when you go up to a girl in a club, totally cased, and just go "Heeeey" and they tweak you're a fair bit intoxicated and just walk off...
EDIT: In her defence though, I was actually 16 at the time, and got into the club on pure fluke of not being ID'd, being an unnaturally advanced adolescent and having a beard that looked like a brown axminster on my face. I got pretty close though
