Not sure if this still works, but if you want to have a bit of fun/really piss people off then tell them (say it gives their name cool colours or something) to set their name or status (in some cases just get them to type it into the chat window) to this string… n—a_Á—ay±m—aÁÇáç±Çáß±Çá§ÁaÇáÇ±OÇá—±Çá§±Çár×ÁÇá+NÇáLáÇá
It buffer overflows messenger causing the user to crash out and not be able to sign back in. Apparently in older versions of MSN it can also crash other chat users who are initiated in a chat session.
Just something random i saw ages ago and see it has started to do the rounds again. Use a third party messenger service such as Adium etc to sign back in and remove offending code.