Me and some friends were paying in Tesco for some things and we chose to go to a cashier who we're all pretty good mates with, so when we're paying one of my friends says to him (the cashier) 'So, are you still working in Tesco?'
I was just gobsmacked, then I laughed a lot. He's never lived it down.
Oh and another one, a few days ago actually ..
I'm going to Benicassim in Spain and I was out with a girl who I know that's going too, she has booked her flights etc and started learning bits of Spanish, and she said to me 'Is Benicassim in Brazil?'. The thing is, she's an intelligent girl, she realised straight away what she'd said but the damage had been done.