This is sort of a reaction to the thread about which language you think in.
When I was little, I learned Finnish, Croatian (archaic), and Swedish from my parents, so that's my starting point.
I can only recall ONE time when I've mixed up language use (unintentionally). In 3rd grade (maybe 2nd?), a classmate in school said my first name, apparently with exactly correct inflection, because I turned around, and said "Sto?" (well, it's a "sch" at the beginning, but I don't know the code for it), that is "What?".
My Swedish classmate was totally lost, and I quickly switched to Swedish.
It's quite possible I've done this sort of thing more times, but since it was a bit embarrassing, I remember that one clearly.
Another variant is the intentional mixing up. Since my Croatian vocab is crappy, I put in German where I know the missing word in that language (since my dad's family's from Austria), and if I'm talking to dad, I take final recourse in Swedish, when I just can't figure out a good word in German. It probably sounds a bit like that guy in Umberto Eco's "The Name of the Rose" ...