If a link is added inside of a collapsible toggle and a user click on it, the target page should be opened. However, in the current implementation of jQuery.makeCollapsible, a click in such links won't open the desired page. Insted, it will expand the hidden element associated to the toggle. See example in the URL above on English Wikibooks and also:
https://secure.wikimedia.org/wikipedia/meta/w/index.php?title=User_talk:Krinkle&oldid=2474984#Collapsible_tables_bugs.3F
https://secure.wikimedia.org/wikipedia/commons/wiki/MediaWiki_talk:CollapsibleTemplates.js#Links_in_header
Version: 1.20.x
Severity: minor
URL: https://secure.wikimedia.org/wikibooks/en/w/index.php?title=Wikibooks:Sandbox&oldid=2076874#Bug:_Links_doesn.27t_work_inside_collapsible_toggles