I'm not seeing these errors all of the time but I have seen it once in a while. Today I got it while doing quests in the Firelands (about halfway through doing the quests so Combat etc is probably not the trigger)

26x Error loading Scrap\Libs\Sushi-3.0\Classes\TextButton.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\GrayButton.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\HelpButton.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\TabButtons.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\TextureButton.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\ColorPicker.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\IconCheck.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\TabGroup.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\Dropdown.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

26x Error loading Scrap\Libs\Sushi-3.0\Classes\TextureDropdown.lua
!BugGrabber\BugGrabber.lua:578: in function <!BugGrabber\BugGrabber.lua:578>

16x FrameXML\RestrictedExecution.lua:431: Call failed: FrameXML\RestrictedFrames.lua:82: Invalid frame handle
[C]: ?
FrameXML\RestrictedExecution.lua:431: in function <FrameXML\RestrictedExecution.lua:420>
(tail call): ?
(tail call): ?
FrameXML\SecureHandlers.lua:115: in function <FrameXML\SecureHandlers.lua:110>
[C]: in function `SetAttribute'
FrameXML\SecureStateDriver.lua:114: in function <FrameXML\SecureStateDriver.lua:95>
FrameXML\SecureStateDriver.lua:127: in function <FrameXML\SecureStateDriver.lua:119>

Locals:
InCombatSkipped