Editing WoW:UI FAQ/AddOns (section)

==== Expanded version ====
Well, there's no guarantee that your AddOns will work error-free after a patch, since it is Blizzard's prerogative to change any and all of the API used by various AddOns. When that happens, AddOns which call upon those API functions tend to either not work, work strangely, or throw up lots of nasty-looking error messages.

The absolute best advice I can give for patch-day AddOn maintenance is to follow these guidelines  

# Remove ANYTHING in Interface/FrameXML and disable any AddOns which provided files that went in there. Way back in the day, the only way for AddOns to actually do something, was hook themselves into FrameXML. This is no longer possible; such AddOns will not work.
# Download the new patch on a patch day.
# Log in to the game and get to the character selection screen. (If prompted about some of your AddOns being out-of-date, choose "Disable".)
# Go into the AddOns screen by clicking the AddOns button in the lower left corner of the screen. (At this point, you should observe that all AddOns are auto-disabled due to version mismatch. If not, then the patch didn't change the UI, and you don't have to do anything. If they are disabled, proceed with these steps.)
# UNCHECK the check box for every AddOn.
# Turn off version checking. (Enable out-of-date AddOns)
# Re-check the check box for any AddOns that you KNOW will work without changes.
# Log in and test that everything works.
# Log Out to the character selection screen and go back into AddOns
# Re-check one AddOn. Log in and test that everything still works.
# Repeat steps 8 and 9 for each AddOn you have. I recommend standing in an Inn to do this - it's faster. (If something kills your system, Kill the wow process via task manager, or coldboot your system if you have to)
# Watch for updates to your favorite AddOns.
# When you see an update, download the update and replace the mod with the new version.
# Repeat step 12 until all of your AddOns are updated.


There is no harm in leaving version checking turned off permanently.

These steps will ensure a clean and aggravation free mod using experience. They will ensure that you don't spend hours trying to figure out exactly what killed your system. When 1300 patch initially came out, the Standalone version of QuestMinion totally locked up everyone's machine, requiring a coldboot. It was a pain to figure out which mod did it.