I had that after the latest update, removing my cache and wtf folders cured it.
Deleting the cache will remove all in-game data that you have seen and has gotten cached for easier viewing in the future (sort of like Temporary Internet Files/Temp). This makes item linking and clicking item links a lot faster and generally makes loading something like NPCs marginally quicker. Deleting the cache will remove these records, though a new one will be created and populated through normal play. For instance, seeing an item link will send it to the cache for easier viewing in the future (sometimes if you fetch an item link through a macro it may not be able to retrieve the data from the server if the item link in question is not "there"). Getting close enough to an NPC to see it will cause it to load up, putting the data associated with the NPC into the cache so you can view it (which is when NPCScan triggers).
Deleting the WTF will remove information associated with characters, whether it is macro data, addon information, or something else. Pretty much everything except for the addon information (saved variables, etc) is stored server-side and is automatically retrieved each time you log in. It is handy to delete the WTF when you want to remove clutter, such as backup versions of files like the macro data or character information associated with other accounts that logged in on your computer. This will not delete addons themselves since they are stored in the Interface folder. This will not delete characters or character information (unless for some reason the servers themselves go severely haywire, in which you will lose your characters no matter what, even if you still have the WTF folder).
Borrowed from
http://www.wowhead.com/forums&topic=209520.1 because I could never have said it so well