It is shown that deviations of estimated from true values of item difficulty parameters, caused for example by item calibration errors, the neglect of randomness of item difficulty parameters, testlet effects, or rule-based item generation, can lead to systematic bias in point estimation of person parameters in the context of adaptive testing. This effect occurs even when the errors of the item difficulty parameters are themselves unbiased. Analytical calculations as well as simulation studies are discussed. (Contains 7 figures, 3 tables and 1 note.)