In an attempt to track down the other issue I'm having with the Quest Watch window clearing itself, I installed !Bugrabber and Bugsack...
Seems this one keeps poping up over and over...and once it starts, I have to reload the UI to clear it...
Seems to start once I kill something dealing with a quest or bonus objective:
Carbonite.Quests\NxQuest.lua:9100: attempt to concatenate local 'color' (a nil value)
Carbonite.Quests\NxQuest.lua:9100: in function `UpdateList'
Carbonite.Quests\NxQuest.lua:8618: in function `func'
Ace3\AceTimer-3.0\AceTimer-3.0-17.lua:55: in function <Ace3\AceTimer-3.0\AceTimer-3.0.lua:48>