Pet usability hate number one - sites that have restrictions on password choice, but don't tell you what the restrictions are. In this case I invented a nice secure password, entered it twice and then was told on submission that the site only does weaker passwords. I don't believe that using special characters is so outrageously unusual as to be something the designers shouldn't have anticipated! Sometime you also see secret rules that passwords must be a minimum length. I think it would be better if any such rules were either unnecessary (so that customers can be protected by the strongest passwords that would work for them), or at least explained before data entry!
Can't say this is the number one hate in terms of things I hate MOST, it's just the first one to come along. Maybe I'll find more "Pet hates" over time. I'm not going to provide links to the sites in question - I don't want to join the "nyah, nyah, ne nyah, nyah your site is rubbish" school of usability writing.