Have you ever seen spectra like those in the top and middle traces below and wondered what was the problem?
These spectra were acquired while the magnetic field was sweeping. This will happen on a Bruker spectrometer if you do not bother locking the field (or fixing the field when running unlocked). On rare occasions in automation, if there is difficulty in locking your sample, I have seem the spectrometer run a spectrum while sweeping the field. This however is very infrequent.