Patch Compatibility
"World of Warcraft" relies on software patches to keep the game running smoothly and to change the game content. As add-ons and mods are created by third parties, they do not have the same release schedule as Blizzard. Updating an add-on or mod to be compatible with a patch can take a lot of time, and that's assuming the third-party developer is still working on that add-on at all. Some add-ons may never get upgrades. Before installing a new add-on or mod, make sure it is compatible with the current version of "WoW."
Resetting the User Interface
If you have outdated or incompatible mods installed, removing them is a good way to prevent instabilities within the game itself. Blizzard Support has recommended a three-step process for restoring your user interface back to the default. After it has been restored, you are free to reinstall your add-ons and mods, provided that they are compatible. The first step in restoration is to delete the "Cache," "WTF," and "Interface" folders from your local "WoW" directory. These folders are recreated every time you launch the game client, so deleting them will not remove any important game files.
The Repair Tool
The Repair Tool is a utility that fixes errors in the game files. Even if you have removed your add-ons, there is the possibility they have corrupted your game files. It is even possible that an updated version of an add-on may have done some damage. The Repair Tool is accessed by launching the "Repair.exe" file within your local "WoW" directory.
Reinstalling Add-ons or Mods
After you've restored "WoW" to its default settings and repaired the game files, it's time to make sure the game is functioning normally. Play the game for a little while before reinstalling add-ons or mods to make sure it is functioning correctly. If everything checks out, exit the game and reinstall your mods one at a time. Though this process can be tedious, you should restart the game after each mod to make sure it is functioning correctly. Sometimes one problematic add-on or mod can cause the others to malfunction.