Speaking at GDC, Eve Online's Distributed Systems Architect, Jacky Mallett, has discussed the difficulties CCP have encountered while maintaining their space MMO. Turns out most of them are down to the players having ludicrous-sized space battles.
Eve Online currently has over 360,000 subscribers and they all log in to the same server. According to Jacky, CCP constantly have to raise their game to put up with player demand: "We're always fighting the real time limits. Every time we reach the point where we can handle the amount of players in a huge fleet battle, they adapt to that and start to increase the amount of players in the battle stressing the limits again. In effect we're always fighting a losing battle there.
"There is one special place where everything works. That's the laboratory, where you don't have enough traffic from users to really stress the system. It confirms our worst suspicions of technical support; we are trapped by the reality that the users will always be able to take down the system."