- enable Hovercards (under "Beta") in your mediawiki account.
- open any page with TOC in mediawiki in "debug" mode (i.e., append "?debug=17" to the address line)
- open JS console ( hotkey: F12 on many browsers )
- click a TOC item and watch the exception report in the console
- disable hovercards, and repeat 2-4. notice that there are no exceptions.
(PS: why does "F12" turns to a link to some file? is there a way around it? )