Classic! We named our guild in EQ "NULL", as a joke. People would comment that we had a problem when they saw us running around with "<NULL>" running over our head. It bit us on the ass when someone went to Sony's EQ con one year, and they literally couldn't print out their badge because "your guild name is NULL? Really?"
It was due to the way they exported the data. They used a SQL statement, and if you weren’t in a guild then the SQL would produce NULL. That output was then used to generate PDF badges, and it tried to do some filtering for NULL guilds. The person writing that script clearly did not anticipate there actually being a guild named NULL.