In 2003-2004 I had a non-SSL IRC server in a German datacenter and found that something between my server and large British ISPs was rewriting all "ISON <nickname>" strings in TCP streams to "PRIVMSG <nickname> :!kapa". I moved the IRC server to another IP address and never had this problem again.

I think that GCHQ was monitoring the network traffic and had a bug in their IRC protocol implementation.