The abnormal behavior of CO E (1)Pi (v=0)<--X (1)Sigma (+)(v=0) three-photon resonance enhanced multiphoton ionization (REMPI) spectroscopy is studied, including the anomalous band structures and adverse pressure effects on the spectral intensity. One- and two-color experiments with laser beams in co- and counterpropagations have been performed to elucidate the mechanisms. It is demonstrated that the abnormalities originate from two mechanisms, i.e., the reabsorption of the resultant third harmonic generation (THG) in the one-color case or four wave mixing sum frequency generation (FWSFG) in the two-color case, and the interference cancellation between the one-photon excitation channel induced by THG/FWSFG reabsorption and the three-photon direct excitation channel. The former mechanism dominates in the wing region of a resonant transition profile, while the latter is restricted within the kernel portion of the transition.