The paper analyzes the occurrence of frequencies with anomalous stirring in a reverberation chamber. Both experimental and numerical field data are checked applying the Anderson-Darling test. The presence of these frequencies is unavoidable, even eliminating evident causes, such as antenna bias. In particular, the parameters that influence this occurrence, such as small perturbations of the relative position of the antenna and chamber quality factor, are analyzed. In the case of simulated field, based on plane wave expansion, occurrences are still present and their variations are related to the change in the field observation points, change in the plane wave set randomly generated, and change in the number of plane waves used to represent t...