When you mention a user on the forum (i.e. @example), the generated URL changes the entire username to lowercase, subsequently breaking the hyperlink to the user's profile page.
Example of this can be found in my comment here.
Note that the first letter of the user's username is capitalized, however the generated URL has the entire username in lowercase so the link leads to a "The page you’re looking for can’t be found", when capitalizing the first letter of the username in the URL, it leads to the appropriate page.
