1) "Mjölk" and "filmjölk" are NOT the same thing, even though they are packaged in very similar looking Tetrapacks. Pouring "fil" into your coffee is a very nasty surprise.
2) It doesn't work to translate "care package" directly from English into Swedish. A care package is a box of goodies your family sends you when go to summer camps as a kid, or later on, to college. But Swedes look at you very strangely when you proclaim exuberantly, "Min mor skickade mig ett vårdpacket!!"
3) Be very careful with long and short vowels. "Att knöla" means something entirely different than what it might if pronounced with a long vowel. (The best translation I can come up with it "to crumple up.") I am never allowed to say that I'm going to "knöla" anything. I use the other word only deliberately, just so there are no misunderstandings about what I want to do and where I want to do it.
4) "Kö" and "kön" are actually two different words, even though "kö" becomes "kön" when you mean to say "the queue." The former is pronounced with a hard k, and the latter is pronounced with a soft k. Imagine my surprise, my second week in Stockholm, when my Swedish friends burst out laughing when I came back from the bathroom slightly disgruntled "Det finns en länge kön i toaletten."
I should also add that I will forever be amused by Swedish words such as utfart (exit) and slut (end, or final). I shall always, always get a smile on my face when I read online profiles whose male authors describe themselves as "en glad prick." I KNOW this means something along the lines of "a nice guy," but I can't help but think that he is referring to himself as a real dickhead. Self-awareness, fellows, is a good thing.