I suspect that every multilingual person has at one time spoken a language other than the one he or she intends to use. There are several manifestations of this phenomenon. One is when a word or phrase from the other language pops into fast, excited speech, as I sometimes hear at work: "So you do this, duì bù duì, and then this..." I also once had a teacher whose (native) Spanish was sprinkled with utterances of the English "you know."
Confusion can also happen when a speaker's audience changes suddenly. I've occasionally spoken Taiwanese to my husband---who understands only English---after conversing with my parents. And I got extremely tongue-tied once at dinner with my Spanish conversation group, when I inadvertently spoke Spanish to the server, corrected myself in English, only to have her respond in Spanish!
Nor does this kind of slip-up occur only when the languages have been mastered. In the beginning of my Spanish studies, I would often struggle to remember a vocabulary word and come up with the German word instead. Now that my Spanish has surpassed my German, the opposite occurs; when I try to speak German, I'll slip into Spanish. I imagine that this is much less likely to happen among dissimilar languages, since I have very rarely used a European language in place of an Asian language or vice versa. But this morning, I was attempting conversation in my very poor Mandarin with a woman who commented that my five-month-old daughter looks a lot like me. In my nervousness I was unable to come up with the translation of "Yes, she does," in Mandarin; what came out of my mouth instead was, "¡Sí!"