Microsoft MSN Messenger allows remote attackers to cause a denial of service via a plaintext message containing the ".pif" string, which is interpreted as a malicious file extension and causes users to be kicked from a group conversation. NOTE: it has been reported that Gaim is also affected, so this may be an issue in the protocol or MSN servers.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-2226
http://www.messenger-blog.com/?p=146