The end of October is a time when everyone in WoW is busy doing all the Halloween events, but it's also a time when thousands of level 1 Gnomes meet up in Booty Bay every year and they all run to the other faction's capital to have some fun.
Around 2500 people showed up for the event this year, organised to raise awareness of breast cancer (more info and donation info here). It turned out to be quite a spectacular march. You don't really see so many people — and so many Chauffeured Chopper mounts — together in the game nowadays.
And even though it was a party of thousands, level 1 Gnomes are not really effective against level 100 horde players or faction boss NPCs.
But it was super-effective to eventually crash the server.