User blog comment:Rappy 4187/Technical Update: February 19, 2013/@comment-166269-20130220011630/@comment-957747-20130220051247

Nested tabbers isn't within the scope of what the tool was created to do. The tabber extension isn't being actively maintained and just happens to still work on MW 1.19. We're looking in to a new extension that is currently maintained. More information soon.