According to the October 2009 edition of Psychology Today (Buy Direct), when we interact with someone we feel physical attraction to, we lose the capability to perform simple cognitive tasks. Worse still, the stronger the physical attraction, the worse things get.
Researchers at Radboud University in Holland discovered this behavioral hiccup - as many people who have suffered from a dating disaster already know - by asking people to take a basic cognitive test after interacting with another person. The more physical attraction, the worse the men's performances, irregardless if the gent was in a relationship or not. Women also fared negligibly, although not nearly as badly as the men did.
Why the embarrassing behavior? When trying to attract someone, a lot of our focus ends up in trying to make a good impression. This leads to small but sometimes significant flub ups with motor control and how we hold ourselves physically, such as tripping when you see an attractive woman or being tongue tied when you run into a hot guy.
The take away? Don't beat yourself up too much if you are a klutz around someone you like, because it happens to most of us (but more so men than women). And for those attracted to men, if the man suddenly starts acting oddly or becomes more accident prone around you, its a good bet he's interested, suffering from this unfortunate situation, and trying his best to make a great first impression. Take it as a compliment, and try to cut him a bit of slack if you can.
Source: Huston, Matthew. "Tongue Tied." Psychology Today September/October 2009 Vol 42, No 5:17.