Weirdly when I try to log onto freenode using ERC, a ` is added to my nickname, which results in the name not being found on the server. I get the following output in the Messages buffer - I have replaced my real nick with nickname.
Connecting to irc.freenode.net:6667... ...done
Logging in as 'nickname'...
Logging in as 'nickname`'... done
Using ERC 5.3 on Emacs 24.3.5.1 on OS X