User blog comment:DaNASCAT/Technical Update: September 18, 2017/@comment-497139-20170919181848/@comment-126761-20170919182219

I believe this is due to some custom JS on your wiki, which may be interacting badly with the updated tabber code - for example, the tabbers work normally on http://leagueoflegends.wikia.com/wiki/Kayn?usesitejs=0 (the parameter temporarily disables custom site JS).

Would you be able to give the custom JS on your wiki a check to see if it's duplicating some tabber JS code?