Several types of failures can occur in the GPS satellites that transmit the ranging signals to the users. Among them, a specific type of failure in the signal generation process aboard the satellite may result in an anomalous waveform being transmitted, called an "evil waveform". Evil waveforms are GPS signals that have a distorted PRN code modulation waveform. The main impact is a rupture of the symmetry of the cross-correlation peak inside the tracking channel, therefore inducing a different measurement error for two receivers that would not have the same loops architecture. As a consequence, there is a potential for evil waveforms to induce large tracking errors of differential systems if left undetected. This paper aims at providing some insight on the behavior of specific ground monitoring techniques that were proposed for implementation.