First in our top 5 is the infamous Winterspring funeral raid. For those who don't know, player interaction is very important to many in MMOs. For some, this can take form in role-playing, where people act out something totally different from their own personality or take on the persona of another character. This funeral was one of those situations.
Since a close friend to many had died, a player decided to host a funeral on the server Illidan, which was a PvP realm, allowing player combat in all neutral zones.
A guild then known as Serenty Now decided to crash the funeral and kill all the players. This caused a huge amount of outrage throughout the community, but it also established an unspoken rule among the MMO community: don't do anything important in PvP zones. It also showed that there are no bounds for the level of jerk people will go to.