There are several catalogues with spectral line calibrators available at the 30m telescope. These catalogues have been compiled using receivers with characteristics that differ from those of the present ones. In particular, the image band rejection is much higher with the present receivers (up to 30 dB for the 3 mm SIS, around 17 dB for the 230 G1 receiver). This means that a line presented in the catalogued spectra may not appear in a newly measured spectrum if the line lies in the image band (which is the upper sideband for the 30m receivers). This fact has recently led to some confusion at the telescope where observers suspected a wrong sky frequency after not having reproduced the calibration spectrum of the catalogue. We advise observers to take the high image sideband rejection into account when comparing new measurements with previous spectra. In order to make the comparison of spectra easier we are in the course of identifying and labelling the lines according to their appearance in the signal or image sideband.