With the globalization of the IC design flow, structural integrity verification to detect parasitic electrical activities has emerged as an important research domain for testing the genuineness of an Integrated Circuit (IC). Sensors like Ring Oscil-lators (RO) have been proposed to precisely monitor the internal behaviour of the ICs. In this paper we propose an experimental analysis of the impact of parasitic electrical activities on the frequencies of ROs and on the internal supply voltages measured. Our observations lead us to identify the limits of the usability of ROs for practical and embedded detection of Hardware Trojans.