Outfitter
Lots of things can go wrong, but one of the most common is people having problems with Outfitter not working at all. Crazy stuff like tons of errors (if errors are enabled), or it won't change outfits, or the UI is a train wreck. This happens pretty much for one reason: Outfitter was updated while WoW was still running. Just restart WoW all the way from the desktop (not just from login) and it'll be happy again. Why? Addons have a file called the Table of Contents which describes how WoW should load and start the addon. The problem is that WoW copies the ToC file into memory at startup and will never update it again as long as WoW is still running. If an addon update has changes to the ToC then WoW won't see those changes if it's still running while you update. Those changes to the ToC may include critical things like new files to be loaded. Without those files the addon likely can't even initialize itself correctly resulting in all those errors, mangled UI, etc.
DB updated: 05/12/2019