A tab could be viewed, edited or deleted through the URL of a different thread, including tabs of threads the visitor could not view
Link tabs accepted unsafe links such as “javascript:”: only http(s) and relative links are accepted now, and existing unsafe links are cleared on upgrade
Attachments of a tab were left orphaned when the tab or its thread was deleted
Tabs were lost when a thread was merged into another one without a redirect: they are now moved...