This paper focuses on the co-adaptation of a group of humans and an intelligent system which acts as a disc jockey. The intelligent system selects and delivers background music in a workplace environment. The humans provide limited feedback about their preferences, and the intelligent system attempts to minimize the number of unacceptable choices of music. During the study, the actual tracks presented to the humans, which humans were present, and the response of the humans are recorded over a 8 month period. The intelligent system (Personal DJ) monitors the responses of the audience, and derives preference rules that describe the mix of music that is acceptable to each audience. It then draws from this music mix, based on the current audience, time of day, and other environmental conditions. The humans have the option of vetoing any song at any time. These veto actions are recorded, and the Personal DJ adapts to these veto actions to refine its model of the audience’s preferences. At the same time, the humans are being presented with a mix of music that is drawn from a wide range of musical selections (rock, heavy metal, reggae, classical, jazz, new age), and during the study period it became apparent that the humans were adapting their preferences to the delivered music. Tracks which were uniformly vetoed under specific conditions during the early part of the study, became acceptable to the same audience under the same conditions during the latter part of the study. This causes the Personal DJ to alter it’s model of the audience and adapt to the new preferences. However, as the new mix of music was presented to the audience, they, in turn, adapted to the new mix and changed their pattern of vetoes, this co-adaptation is capable of causing a pattern similar to Pilot Induced Oscillation. The paper presents an analysis of this phenomenon, and draws some guidelines for the design of adaptive systems which will have long-term interaction with humans.