If a user gets so confused she doesn't buy or sign up, if she gets so frustrated she tells others to avoid the site, or if she can't figure something out and never comes back, we've got a serious problem. Luckily, user testing can help avoid such situations. Something that looks good on a white board or on a designer's iMac may not be clear to the average user. But if you don't sit a real user in front of the screen and watch his brow furrow in consternation, you may never know. Often we forget to make things easy for those who aren't web savvy.
Small things can add up to big confusion if ignored.
Do this for all fields where the input could vary, such as date formatting. If you're asking for information that seems incongruous or requires some specific knowledge, provide a link to clearly define the question. Every chance to confuse a user is a chance you may lose the user. User testing can improve site usability and functionality. One web site had a problem with its login screen.