Production planning decisions are usually made by human planners that are assisted by decision support systems. While it is widely argued in the literature that current decision support systems for production planning are generally inadequate, it is not clear to what extent human planners actually disregard the planning decisions proposed by the system. In this study, we investigate this question. In a setting in which the planning system's model has an adequate representation of reality, we collect data on actual planning decisions and compare them to the planning decisions proposed by the system. We conclude that planners systematically and largely neglect the system's recommendations and that the extent of neglect is larger if the planning problem is more complex.