Fault injection attacks can be carried out against an operating circuit by exposing it to EM perturbations. These attacks can be detected using embedded digital sensors based on the EM fault injection mechanism, as the one introduced by El Baze et al. [1] which uses the sampling fault model [2], [3]. We tested on an experimental basis the efficiency of this sensor embedded in the AES accelerator of an FPGA. It proved effective when the target was clocked at moderate frequency (the injected faults were consistent with the sampling fault model). As the clock frequency was progressively increased, faults started to escape detection, which raises warnings about possible limitations of the sampling model. Further tests at frequencies close to the target maximal frequency revealed faults injected according to a timing fault model. Both series of experimental results ascertain that EM injection can follow at least two different fault models. Undetected faults and the existence of different fault injection mechanisms cast doubt upon the use of sensors based on a single model.