I received an email notification from Confluence with an unsubsituted value.
The notification was of a comment replying to another comment.
The unsubstituted value was the name of the original author.

In reply to a comment by $action.getUserFullName($username):

Possiby relevant (but probably not). I had a watch directly on the page. I was the author of the original comment. The notification was for thishttp://confluence.atlassian.com/display/CONFEXT/Copy+Space+Plugin?focusedCommentId=97911549#comment-97911549.