Just completed the quest to earn the Highland Drake mount. Upon trying to use/learn it, got a popup that "AllTheThings has been blocked from an action only availble to the Blizzard UI".

No LUA error popped up, but BugSack captured the below error message:

1x [ADDON_ACTION_FORBIDDEN] AddOn 'AllTheThings' 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"]:1409: in function `ContainerFrameItemButton_OnClick'
[string "@FrameXML/ContainerFrame.lua"]:1461: in function <FrameXML/ContainerFrame.lua:1448>

As is often the case with these "blocked from action" messages, a UI Reload without disabling any addons cleared it.
But figure I'd report it in case it's related to something bigger.

As is likely the case, some other addon is using the same code library called 'LibStub' but since ATT used it first, all of it's code if linked as 'ATT's code' (I think this is what happens anyway...)

I don't believe ATT has any 'when clinking an Item' logic linked anywhere, so this being actually ATT's fault seems unlikely. Hopefully it hasn't been happening since Nov :)