I have some biomedical signals. If I calculate their magnitude spectrums, I see power-line interference at 50 Hz. I was excepting a harmonic at 100 Hz, but it turns out there is none. However, there is a harmonic at 150 Hz. Is this common or should I worry that I have done something wrong?
Yes, this is common. Common non-linearities (such as op-amp clipping) preserve the symmetry of the upper and lower section of the waveform; and as such, they do not introduce even harmonics - just odd harmonics.