Scoobster - well at least you quickly spotted your mistake and quickly dealt with it, so you should at least be proud of yourself for that :-) . We ALL make mistakes!
I have heard about similar situations though, like pilots thinking that they were retracting flaps, when they actually retracted the undercarriage on the ground ... ooops. I am hoping to do my CPL this summer, and definitely hoping not to make that mistake!
I know what you mean though about making silly mistakes because you are nervous. Recently on my first flight in a new a/c type, I read back the wrong runway on the first couple of circuits. I was flying circuits for the correct runway, but I read back the wrong runway, I put this down to being anxious about flying the new aircraft. A few more flights later and i've done a few local trips in it, and would now be happier to take the a/c further afield. But I still feel happiest in a little 152!