Some web sites don't want you create accounts like
me+1@gmail.com
me+2@gmail.com
me+3@gmail.com
when it comes to making an army of sock puppets the biggest hassle is usually doing the email verification for all the accounts. The "+" trick greatly simplifies this though if somebody is looking for trouble they sure will find it. Of course if you control your own domain you can use a wildcard but if you add 1000 users to a site from the same domain that is obvious too.