This paper deals with the External Model Repetitive Controller, a structure that combines the classical repetitive disturbance model (in the form of a time delay with a unit feedback) with a feedback of a disturbance estimate. The latter is often termed the External Model Controller and has a broad spectrum of technical applications, e.g., active vibration control. By analyzing the controller performance under the assumption of a disturbance period modeling error, it is shown that an unacceptable performance degradation might occur even for very small values of the modeling error