In this paper we describe one of the series of experiments with Extreme Programming, carried out as a summer project. The focus in this experiment was to try out the XP customer model and Design by Contract. The experiment indicates that the Extreme Programming emphasis on having an on-site customer available during the project improves the communication between customers and the programming team, and markedly decreases the number of false features and feature misses. It also indicates that the systematic use of Design by Contract leads to a low post-release defect rate for the software system built.