What was the most embarrassing cultural blunder you've ever made while traveling to another country? Did the locals explain your mistake to you or did you find out the hard way?

Here's my story: my father fell in love with Japan while he was in the Navy, and kept studying Japanese in hopes of living there someday. I'd look through his textbooks and look up words I wanted to know.
My father moved to Japan after my parents divorced, so when I was 10, I traveled to Japan for the summer with my brother and sister, unaccompanied by any adults. While waiting on line in Narita Airport, my little brother became fidgety because he had to go to the bathroom. We couldn't leave the line though, and he soon started attracting attention as he crossed his legs and squirmed. One very dignified elderly Japanese lady turned around and asked me what was wrong with him.
He just has to go to the bathroom, I told her. I used the word for bathroom that I had seen in my father's Japanese textbook, "benjo".
The elderly lady looked at me as if I were a piece of trash and turned around in a huff.
I didn't realize what I had said until later that summer when my dad bought a blackboard and workbooks and began to teach us Japanese in earnest. He taught us the polite word for bathroom one day: "otearai", which meant "honorable hand-washing place". I told him I thought the word for bathroom was "benjo" and he stared at me. "Where the hell did you learn that word?"
I told him I had looked it up in his textbook, and that solved the mystery. Apparently his Japanese textbook had been written in the 1950's by sailors. The word I'd learned for bathroom literally meant "shithouse".

When i was in the military I once ate with a traditional Aghan village. They brought out a mix of rice and lamb in a big giant bowl and gave everyone a naan (flat peice of unleaven bread).

So everyone was just reaching into the bowl and grabbing the lamb and rice and putting it on their naan.. I kind of watched for a little bit for going in to grab the food myself.

All of a sudden everyone in the room gave me this wretched look and they grabbed the bowl and started dumping out the area where i touched the food. I was confused as i thought this was what everyone else was doing.

One of the terps I knew came to me and explained that I had grabbed the food with my left hand and the right hand is for eating. Reason being, the left hand was the hand you wipe your butt with.

lol... felt like a retard but after that i made sure never to touch anyones left hand.

First time to a Japanese restaurant -- picked up the whole dollop (?) of wasabi on my plate and put it in my mouth.

I have this theory that the Japanese really don't need a standing army when they have that weapons-grade wasabi And when it comes to Japanese cuisine, the smaller the deadlier!

wow!! and you lived to tell about it!