Selenium WebDriver: Target server failed to respond error

I am writing some junit tests using Selenium WebDriver.The tests run fine. But then sometimes I get "Target server failed to reespond" error and the tests fails.
Can anyone please explain me what this error means, why does it come in such a random manner, and where to look to fix it?

I think you might have better luck getting answers in the Testing forum, I'll move your post for you (in such a way that is still appears in this forum also - you'll get twice the eyeballs).