As I've seen in WoW, a player is using a program that allows him to control one single character, while all the other characters following him like rats do the exact same attacks. It's clearly synchronized and not a player behaviour obviously, it's a bot controlling all of them.
See the pictures below (safe links, no worries):
http://image.noelshack.com/fichiers/...9-01-00011.jpg
http://image.noelshack.com/fichiers/...9-01-00012.jpg
http://image.noelshack.com/fichiers/...9-01-00013.jpg
http://image.noelshack.com/fichiers/...9-01-00014.jpg
http://image.noelshack.com/fichiers/...9-01-00015.jpg
http://image.noelshack.com/fichiers/...9-01-00016.jpg
http://image.noelshack.com/fichiers/...9-01-00017.jpg
http://image.noelshack.com/fichiers/...9-01-00018.jpg
http://image.noelshack.com/fichiers/...9-01-00019.jpg
http://image.noelshack.com/fichiers/...9-01-00020.jpg
I don't name&shame but if a GM wants the name... It just happenned on Crom by the way. Kinda sick to use cheats right in front of everyone.