Stimulated Brillouin scattering can cause asymmetries of ¿25% and ¿1% in the double humped spectra of incoherent backscatter measured at Jicamarca and at Arecibo, respectively. Errors of 10--15 m/sec in the measured velocity could have been caused by ignoring the effects of these asymmetries at Arecibo but the typical errors were probably much smaller.