Fault tolerance is an essential requirement in the design of protocols and applications for Wireless Sensor Networks (WSNs) since communication and hardware failures are frequent. In this paper we studied the resilience of routing protocols for continuous data dissemination WSNs in face of faults. The main causes of silent failure are presented and classified, including security attacks. An evaluation of routing protocols shows that failures under a large region of the network are the most damaging. We also show how routing protocols may save energy by temporarily turning off disconnected nodes.