Once every 24 hours, every game world has to save the changes in characters' stats, levels, items, etc. Before such a server save can take place, all characters in the game world have to be logged out. Therefore, you may get notification messages like:
hh:55 Server is saving game in 5 minutes. Please come back in 10 minutes. hh:57 Server is saving game in 3 minutes. Please come back in 10 minutes. hh:59 Server is saving game in 1 minute. Please log out.
hh: hour of server save of that server
As soon as you see the first notification message, you can log out at any time, even in case of a logout block. Before 2006 summer update, on normal PvP-worlds, you can attack a player, thus get a skull, and log out one second later. Needless to say, this makes server save a very dangerous period.
But now, Cipsoft has changed the rules for logging out before the Server Save. Players can now no longer log out with a white skull unless they are forced to by the server shutting down.