  We report here the discovery of a significant source of systematic error in
the rotation measure determinations of pulsars. Conventional analysis of high
sensitivity polarimetric observations of PSR B2016+28 display variation of the
rotation measure of $\pm$15 rad m$^{-2}$ (around the mean value of -34.6 rad
m$^{-2}$) across the pulse profile. Analysis of single pulse data shows that
this variation is an artifact of the incoherent superposition of
quasi-orthogonal polarisation modes along with the frequency dependence of
relative strength and/or quasi-orthogonality of the modes. Quasi-orthogonal
polarization is common among pulsars, and therefore this effect needs to be
taken into account in the interpretation of pulsar rotation measures.
