Am I the only one who gets irritated when asked that??
N plus, usually your music tastes aren't the same as the people who ask you, so after your half-hearted performance they'll give you a poker face n say, "Oh that was cool, but can't you play blah-blah-blah?"

I think this was mentioned in Point #7 here http://leftyfretz.com/top-reasons-to-play-guitar-left-handed/

I hate it yeah, but I've got a pretty good excuse for getting out of it at the moment

People soon get bored if you keep running up and down diminished and whole tone scales while convincing them it's all you do. On the other hand if it's a girl asking then play what she wants to here

Reporter: Can you sing something for us?

Lennon: No, we need money first!

- Beatles 1964 press conference, JFK airport

I had that request one night when I took the Tele out to a club where some friends were playing. After the show the band was looking over the guitar, and the lead singer asked me to play something on it so he could hear how it sounds. how do you say no to a guy who's been playing for you for 30 years? So I plugged in his cord planning to run through my Something solo, and wouldn't ya know it... the sound man had powered everything down.

Lucky for you!
I got cornered at home with an acoustic in my hand

haha, yeah this is never cool.

I quite like it when my mates pick-up a guitar and suddenly realise it's all backwards though It generally means that playing ANYTHING after their attempts goes down well!

Haha yeah, the easy songs like wonderwall or good riddance always seem to work best.

Can you guys also play upside down on a right handed guitar? I play better upside down than my righthanded friends, probably because I come across right handed guitars way more often than they see lefties. Nothing fancy though

Almost every LH player can work a righty guitar better than a righty player with a LH guitar. As you say, we get much more practice than they do (usually none).

I'd told the tale before of playing someone's RH bass at a jam party while he took a beer break. The look on his face when he walked back in and saw me playing his bass, upside down, was amazing.

I once subbed in for the RH bass player of the band of a friend, because the bass player was on holiday. I played the gig with the bassists gear, so bass upside down . Afterwards they asked me if I was interested in joining the band, because I was better than their usual bass player ...cracked me up