Maybe this is more about the power of suggestion
0. UK, where pants is pants and not trousers. ;)
Edit: Also, pants is a UK way of saying rubbish. So, "that web site is a load of pants", might be said. Might be a sense of humor at work here.
Pro-tip: Telling your local UK colleagues that you were running late because you stained your pants and needed to get new pants, and then offering to show people the stained pants is going to lead to some awkward moments.
"Fanny" is a great cause of transatlantic fun too. And if you are a Brit in the US, never ever say, "Im diving out for a fag". You're gonna get some odd looks.
I made this mistake once when I spent a summer in Ireland in high school. (I should add here that the people at this summer program were noticeably more homophobic than I was used to back home[0]) I had borrowed a pair of "trousers" from a friend for a costume for some event.
When asking permission from the RA later that night to go to a different dorm after lights-out, I told him, 'I need to give <friend> his pants back'.
I will never forget the look of horror that he gave me.
[0] Not sure if it's a cultural thing or just the program I happened to be at, but either way, it's relevant.
As a side note, I saw undies in the original too!