languages

I've heard so many people telling stories about how they forget that they have to speak in a particular language with different people ... and there it came my turn; quite funny for me.
First it happened 2 or 3 months ago while chatting with a romanian: it was english, than french and in the end a few words in romanian. My first reaction on seeing the romanian words: errr, in what language are you speaking?
After that an "aici" slept out of my mouth while explaining something to my boss.
Last week, I did it really great: "Do you want ceai, Mihai?" ... I didn't realized what I was saying until he begun laughing at me :)
And today, another bird flew out of my mouth "poftim?" (with a large smile to my boss).
I haven't paid very much attention to speak in a language or another; it is easy for me to control it (even though sometimes I really feel like talking 1001 words per second in romanian). It really amuses me to hear myself speaking in the wrong language ... hmmm ... I think I will try sometimes to speak suddenly only in romanian to somebody who doesn't know the language; I want to see the expression on his/her face :) ... any volunteer?