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In addition to detector fringing, the VIMOS IFU suffers from additional, not reproducible fringing affecting some pseudo-slits. It can severely decrease the sensitivity of the instrument. Its origin has not been clearly determined. Here, we present the analysis of calibration data obtained in October 2006. We show that fringing originates either from inside the mask at the pseudo-slits level or from an imperfect positioning of the masks themselves. The lack of reproducibility of the fringes may be caused by the imperfect fixation of the prisms in the pseudo-slits or a loose positioning of the masks.