2x [ADDON_ACTION_BLOCKED] AddOn 'Mapster' tried to call the protected function 'MultiBarLeftButton12:SetAttribute()'.
[string "@Interface/AddOns/!BugGrabber/BugGrabber.lua"]:480: in function <Interface/AddOns/!BugGrabber/BugGrabber.lua:480>
[string "=[C]"]: in function SetAttribute' [string "@Interface/FrameXML/ActionButton.lua"]:355: in function UpdatePressAndHoldAction'
[string "@Interface/FrameXML/ActionButton.lua"]:409: in function Update' [string "@Interface/FrameXML/ActionButton.lua"]:797: in function OnEvent'
[string "@Interface/FrameXML/ActionButton.lua"]:216: in function <Interface/FrameXML/ActionButton.lua:213>

Mapster does not interfer with the Action Buttons - directly in any case. That said I have removed a source of taint that may (or may not) have spilled over here in the latest alpha version, so going to close this and will revisit if it still happens.