Happened when I used my hearthstone. Shrug. At the time I had the Bagnon module enabled (but Bagnon was disabled due to bugs) and I did not have the Blizzard module enabled (tried that after and got another error, will report that in a sec).

6x [ADDON_ACTION_FORBIDDEN] AddOn 'ItemLevelDisplay' tried to call the protected function 'UNKNOWN()'.
[string "@!BugGrabber/BugGrabber.lua"]:480: in function <!BugGrabber/BugGrabber.lua:480>
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: in function UseContainerItem' [string "@FrameXML/ContainerFrame.lua"]:1402: in function <FrameXML/ContainerFrame.lua:1311> [string "=[C]"]: in function ContainerFrameItemButton_OnClick'
[string "@FrameXML/ContainerFrame.lua"]:1454: in function <FrameXML/ContainerFrame.lua:1441>