I'm creating this bug per https://en.wikipedia.org/wiki/Help_talk:Notifications/Archive_4#Why_wasn't_I_notified... where-as it seems that custom signatures that are missing closing tags are causing chaos. I'm thinking the best thing to do is to run a parsed version of the signature through http://validator.w3.org and refused with a little red error message on the preferences page saying "Invalid raw signature. Check HTML tags." and then use the default signature instead.
Version: unspecified
Severity: minor