How many times have you gone to register at a company's website (something they try to force you to do), only to have to fill out the website's forms with all kinds of information, only to be told AFTER THE FACT that your username, password, etc has to be in a certain form. Ridiculous.
Here's an example from an insurance company. They tell you to type in your policy number exactly as it appears on your paperwork. When you do that, you find that you can't enter all the digits! Only after a few more minutes of research and trying do you find out that, in fact, you should not type in the dashes. So why tell the user to "type in your policy number exactly as it appears on your policy" if that policy number includes dashes?