You can take it at face value, ie. "it has been a nice experience knowing you". Used as a farewell of sorts, much like "see you later". But (like "see you later") it can be used as a sarcastic brush-off. You're saying good-bye when you mean good riddance. As far as I understand it, you can use it in either way. The only real way to tell the difference is from the tone of voice and the context.

It is slang and your guess is correct: Someone is about to get in trouble. It's a joke, something you might say to someone who broke something that belongs to the boss! —usually used only when *real* trouble isn't expected. It can be a hurtful phrase if the person is really worried about the problem. The meaning of the phrase is you're never going to see the person again. They're going to die or go to prison!

["Nice knowing you" isn't slang]<-(ignore) I've never used it as "nice to know you / nice to meet you". It's usually sarcastic or used as a joke, in my opinion. Example: Person A - I'm going to go travel the world for the rest of my life. Person B - Haha, well, it was nice knowing you! OR Person A: I'm going to go jump out of a plane tomorrow! Person B: Wow, nice knowing you. EDIT: Actually, I agree with Scott Larsen. I guess I just never thought of it as slang, haha.

"Nice knowing you" simply means "it was nice to meet you." "Nice knowing you" can also be used in sarcastic manner when someone is about to rid the person they're speaking too. Imagine a shooting video game online and someone says "nice knowing you! HAHAHAHA!" and then defeats the other player.

"Nice knowing you" Isn't slang, It's another way to say "Nice to meet you" It really means "It's nice that I know you" or "I like that I know you"