Ooooooh... and then there was the time I was crowing to my friend's flatmate about the fully home cooked meal I cooked - I'd even made garlic bread from scratch, well not the bread, mixed butter and garlic, more like!
Felix was laughing at me about it - "Did you grind the flour yourself?"
Me (remember, despite my gender bender acronym, I'm a girl) - "Yeah! I even cultivated the yeast!"
I am like Firas - you'd have to ask my friends for a detailed, annotated listing, but the one I will never, ever live down:
Freshman year in college, while working in a physics study group: "Has anyone ever actually seen a photon?" The resident physics major gave me this incredulous look, then pointed at the ceiling lamp over our heads. *palmface*
Most of my best involve language barriers. I am a native english speaker, but speak some Spanish as well. I was in Havana at Carnaval and was being harassed by some guys. I wanted to tell them to leave me alone so i yelled QUITAME! Apparently that actually means TAKE OFF MY CLOTHES! in Cuba. As you can imagine, this did NOT discourage them.
Then there's the time I tried to get diarrhea medicine in a cosmetics shop(I thought it was a pharmacy) in thailand. I don't speak any Thai. Have you ever tried to mime diarrhea? in a makeup shop? I assure you that hilarity ensues.