A lot of websites have login forms. Those usually have two fields: one for the username and one for the user to enter his password. Pretty simple, you might say, but that's exactly where things can go wrong.
Login forms are usually so simple they're not considered 'real' forms in terms of how they should look. A lot of websites hide their login forms in a corner or behind some sort of button. As a consequence element labels take up too much space–bigger forms means more effort to hide them in a corner. So, let's remove the labels then. Voila! We have a very small form. Just two input elements and a button. But wait, we can even remove the button, because forms can be submitted by hitting return, right? Too bad not everybody knows how to submit forms this way.
Let's summarise: don't prepopulate login forms. Ever. And if you start wondering if users will still get how to use your form, consider putting back the labels. After all, shit don't just have to look good, but it has to be usable as well.