A study was carried out which examined the extent to which young people aged 11-12, with no prior instruction in programming, are able to write computational rules which govern play in a 3D computer role-playing game. Expressing these rules required the use of common computational structures such as conditionals, sets and loops. We analysed the rules written for their structure and style, and recorded the types of errors made. It was found that although young people were able to abstract away from the game play experience, very few of the rules were error-free. The most common errors were errors of omission (leaving elements out that should have been included) rather than errors of commission (including elements which should not be part of the rule). These findings have implications for the design of the Flip language, which aims to support young people as they begin to develop computational skills through game design.