As a Norwegian living in Sweden I wouldn't have to speak Swedish, but I do it anyway and I'm pretty good at it. Sometimes the mistakes sneak up on me though, usually in the shape of how it is said in Norwegian. And usually this is passed off as cute and adorable.
There is this one mistake though, that I keep making, and it is horrible :P. I'm not gonna bore you with the details, but basically I make "I hate those sounds!" into "I hate those Jews!". And usually when I make this mistake I have no idea that I made it until I see the faces of the people who heard me say it. I remember the first time was with Anders and he just stared at me like O_O. That time in the subway was interesting as well :P