2. Sound mouse-overs.
Unsurprisingly, this gem comes from the creators of the 5 second page transitions and if from the same website.
On the main navigation links there were sound mouse-overs, which weren't accompanied by any form of visual mouse-overs. So, if you had your speakers turned off, or were deaf...then there were no mouse-over effects.
Sound mouse-overs which are accompanied by visual mouse-overs are generally just annoying, but without visual mouse-overs, they're a major usability and accessibility problem.

1. Poorly done CSS drop-down menus.
CSS drop-down navigation menus can be really cool, and much better than any form of Javascript or Flash alternative. But in some cases they're kind of finicky, and will close as you're moving your mouse over them. Or, they're just too small and you accidentally move your mouse off them, causing them to close.
In the worst cases, menu items can prove impossible to access, because as soon as you move your mouse towards the desired link, the menu closes each and every time.

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