Its always the times that you care the most about making a good impression that you find yourself falling flat on your face. Especially when trying to make a good impression with the opposite sex. I can't tell you how many stupid things I have done when I get nervous.
One time in college I was walking down the hallway with a guy that I was very into at the time; his opinion of me was always at the forefront of my attention if you know what I mean. As we were walking we came to a doorway and I grabbed the handle to open the door for us. I struggled, pulling on the doorknob for a while, only to realize suddenly that it was indeed a push door. Not intended for pulling. I was so mortified in anticipation of him mocking my stupidity that I turned around and did the first thing that popped into my head---
I slapped him across the face.
Naturally... isn't that what you would have done?
No? Yeah, well the poor boy hadn't even muttered two words or had the time to make fun of me but apparently I was such a wreck under the pressure of his opinion that I succeeded in convincing him of my absolute lunacy.
Come to find out later that he had a huge issue with people slapping him. Who knew?
That didn't last long.
Another time when I was living in Philadelphia a friend of mine had me take his pilot buddy out for a night on the town. Here is an excerpt taken from my journal:

"May 15, 2005:
Last night Jon called me to tell me his buddy Matt was in Philly and bored out of his mind. So I called Matt up and told him I'd pick him up from his hotel. I thought 'No big deal, right?' That is, until he got in the car and I saw he was drop dead gorgeous.
Uh oh.
I froze like a deer in headlights. I forgot how to speak and whats worse, how to drive.
I was never that good to begin with.
First, I pulled out of the hotel onto a one-way street, going the WRONG WAY into oncoming headlights. I started screaming and he grabbed the wheel and jerked us over the median.
'Great, I've known this guy for 30 seconds and already put his life in serious danger'.
Not a great start, I'd say.
Then less than a minute later I was trying to redeem myself by creating some distracting conversation so he would forget about the 'incident' when !BAM! I blew through a stop sign and almost killed him again.
(He smelled really good, did I mention that?)
Needless to say the rest of the night did not go any better. The movie was a bomb too.
In the middle of the movie there was a scene where the girl is holding flowers. I turn to him and say, "Oh, those are my absolute favorite flowers in the whole wide world!"
He says, "Really? What are they called?"
"Uh...hmm. Good question. I think it starts with an 'A'?"
Wow, genius.
Driving home I took 95 north instead of 95 south, managed to get us lost, and to top it off at the end of the night he started talking about his girlfriend.
What? You mean to tell me I just made a complete fool out of myself...for nothing!?"

The worst is when it is so obvious that you are nervous because you are tripping all over your words. There's no hiding that.
Like the time I declared, "Well, you know what they say: It's all down there from hill!"
Recently I was out on a date when the guy I was with asked if I owned or rented my place. A fairly simple question to answer, right?
My answer went something like this:
"Uh, we own it. Actually, rent it. No...my roommate owns it. Not 'we'. I don't know why I said that. She owns it, I rent it to her. Well, no...I mean for her. Actually from her... I rent the room from her. The house is hers and I pay her money to live there".

Do you see how difficult this is?