I think my regional dialect of Portuguese has got to have the most confusing expression for outsiders ever: our use of "capaz" which literally means "capable". We use it like this:
— are you going to the party tomorrow?
Here's the kicker: "capable" means "no"!
It makes people from other Brazilian states *very* confused
We also use "bem capaz", which literally means "very capable", to mean "of course not!"
People from the rest of the country, trying to make sense of what to their ears sounds like a sentence fragment, often assume it to be a shortening of "é bem capaz que sim", which means "it's quite possible that yes", which means the exact opposite!
It comes to mind now also that we also use it as a possible answer for obrigado/"thank you", say, if you do someone a favor, as a way of saying "nevermind, it was no hassle!"
And now I can't help but find this exchange hilarious:
— thank you!