Effective error detection is paramount for building highly dependable computing systems. A new methodology, based on physical and simulated fault injection, is developed for evaluating error detection mechanisms. Our approach consists of two steps. First, transient faults are physically injected at the IC pin level of a prototype server. Experiments are carried out in a three dimensional space of events, the location, time of occurrence and duration of the fault being randomly selected. Improved detection circuitry is devised for decreasing signal sensitivity to transients. Second, simulated fault injection is performed to asses the effectiveness of the new detection mechanisms, without using expensive silicon implementations.Physical fault injection experiments, carried out on the server, and simulated fault injection, performed on a protocol checker, are presented in this paper. Detection effectiveness is measured by the error detection coverage, defined as the conditional probability that an error is detected, given that an error occurs. Fault injection reveals that coverage probability is a function of fault duration. The protocol checker significantly improves error detection. Although, further research is required to increase detection coverage of the errors induced by short transient faults.