There was an old campaign, a hodgepodge of material stolen from every imaginable location, being torn apart by two DMs with opposing styles.  So it was decided that I, the DM the majority of the time, would start a new campaign with the same characters.  This caused some problems as a few magical items had be distributed while I wasn't watching, but nothing I couldn't deal with.
   So, I decided to conduct an experiment.  I had each player name and design the orginization their character belonged too (the characters were the same, but they had no ties to the old world).  So Matt designed the Order, an orginization of Druids and rangers occupying a huge forest.  Jim named his homeland the Midlands (Terry Brooks reference) and decided he was an independant paladin under orders from the king.  Paul decided he belonged to an order of psionicists in the mountains, but left me decide the rest.  So the Illuminati were born (I couldn't resist).
   After a few minutes it struck me.  I had the world they were describing in one of my folders from a few years back.  Just something I had designed to releive boredom.  So, I pulled it out.  The great forest's guardians were renamed the Order, Kor's Land was renamed the Midlands, and a tower filled with psionicists were plopped into the Windknife mountains.  It was so easy I was amazed, Kor (King of Kor's Land, imagine) was even a powerful paladin.
   Thusly it began.

    Moral of the Story: Never, ever co DM with someone that you don't have a good understanding with.