The influence of harmonics contamination and geometric misalignment is discussed. It is shown how harmonics contamination affects X-ray absorption fine-structure (XAFS) spectra. One of the most important suggestions to avoid harmonics contamination in XAFS analyses is to reduce the third-harmonic contamination level to 10⁻⁵ of the fundamental. If possible it should be reduced to 10⁻⁶ of the fundamental, and at least to less than 10⁻⁴. Slits and apertures must be set at their correct positions. Otherwise, the absolute energy of the incident X-ray shifts and the resolution becomes worse.