Links ending with a dot will 'loose' this dot when presented in an email (such as verification mails!). See my posting to this case and its makeshift solution.
The function "get_size_url" in /wp-includes/link-template.php (and any other responsible function) should account for this in future versions, by example changing the position of the request paramters, or adding a dummy paramater if the link ends with a dot.
Otherwise, such links (b.e. usernames ending with dots) may cause errors when clicked on.