Harmonic distortion and Nyquist rate (Spurs above the Nyquist rate of a DAC)
The spurs are generated by small nonlinearities in the ADC itself, after any possible input filtering. Yes, the spurious frequencies generated can be higher than the Nyquist rate of the system, in which case, they'll be "aliased" to some other frequency. They still represent undesired contamination of the signal data.