This really is a tricky one:
Create a new page or edit an empty page. Try to mention someone. Now add at least one empty line and below that line your signature. The mention will not be send. Also no mention failure notification is sent.
Looking into the code this seems to be an issue when parsing the diff. When adding content to an empty page that contains an empty line, the content before that line will treated as separate addition than the content below the line. The DiscussionParser then does not associate mentioned user and signature to trigger a mention.
Create a new page/edit an empty page with:
It works when ping and signature are below the first empty line:
First line [[User:UserToPing]] Content ~~~