We were talking in the Translation thread about how hard it can be to use a computer translator to make sense of idiomatic or slang expressions. I'm sure all of us who speak English have more than a few that we know have baffled people trying to learn our twisted language. I'd be interested to hear a few from other languages, as well.
So I'll start with my favorite German expression my Opa von Steiner used to use on me: "Aufgehe!" If you put it into a translator, you'd probably get something on the order of "Get out!" Really, it means "Get up! (out of bed)".
My favorite English nonsense expression is "by the way". It can mean "oh, while I'm saying this, I'll add something", or "just incidentally, I noticed (fill in the blank)". But you can also say you left something (or someone) "by the way(side)", meaning you lost it or abandoned it. And you can also use it to mean you actually found something lying next to the road. Which leads to "stumbled upon", which isn't the same as "stumbled over"...
Oh, we could go on in that vein forever. But what's your favorite confusing phrase you know from your own language or from having been confounded by it in another?