Ever since I've been using SO, I've always 'known' questions that have a favorite tag to become highlighted (their background becomes yellow in the selection list). However, recently I noticed one of my favorite tags wasn't working.
A little search based picture to make it a little more obvious:
As you can see I also have no ignored tags, so I don't think there is a conflict there. Is this a bug, or is this functionality added by something else?