When a user adds
text text text == section header == some more text
and the text parts contain user links, these should result in mentions, if in the section in question a new signature was added.
One wish of the German-speaking community wishlist 2015 was to be informed about mentions you sent (T139962)
We decided to include notifications about mentions that failed to send, too. One failure, the "multiple section edit" failure, occurs, when people add text and a new section, where the header is not the first item in the changes. This failure blocks all notifications from this edit, no matter if they were part of the section or not. Especially, since mentions are hard to correct (you would always have to add a NEW user link and signature), we decided to fix that issue to have more notifications be sent.