I have a toolbar that displays my Favorite Locations. When I hover over most of the favorites that appear in the list, the folder icon for that favorite will animate... it appears to open and the shading changes. This is true for all of them except for a few... those won't animate but the shading changes.
If you look at the attached .swf file, you can see that the icon for the 'ERP' favorite doesn't change. Bug? bad favorites.zip (350 KB)
It may have a customized folder icon (set to the same icon that normal folders have) which would mean it doesn't open. Check the Properties dialog for the folder. If it is that you'd see the same thing in the folder tree where the icon doesn't change when the folder is selected.
(This is because if you customize the icon you can only specify a single icon; there's no way to specify a custom "open" icon for individual folders, at least not through the standard Explorer Properties dialog, as far as I know.)