Titles like ".dm" ( http://sah.wikipedia.org/wiki/.dm ) will throw an error when using the mw.Title constructor with them. Apparently mw.Title tries to be smart and treads this as a title with file extension. At least it enters mw.Title.setNameAndExtension() where the validation fails. '.123456' though will be accepted since the part behind the dot is longer than 5 characters.
Version: 1.19
Severity: normal
Yeah, it shouldn't validate extensions because extensions are only an end-user convenience, no significant to them.
rephrasing summary to problem-solution orientation.
It fails because after stripping off the extension, the title is empty, which is invalid.
Change 83047 had a related patch set uploaded by Krinkle:
mw.Title: Rewrite from scratch (porting logic from Title.php)
Change 83047 merged by jenkins-bot:
mw.Title: Rewrite from scratch (porting logic from Title.php)