Usually I end up learning new slang using words that already exist.  For example, I just learned that "salty" can be used to describe someone who is annoyed.

As for surprising things, I learned that the double contraction "I'd've" (I would have) is actually proper English.  It's not something you ever see typed, but you say it a lot and you don't realize it's a word.

I learn all kinds of new words from people learning English here on italki. I probably have to look up at least one new word a day. Even sometimes they are words I recognize in context, but I don't actually know their definition until someone asks for it to be explained. The most recent one I learned was "rote" as in rote learning which means memorization I guess. Who knew?? Not me apparently.

Thanks for challenging us to better learn our own language!