AFDX (Avionics Full Duplex Switched Ethernet) is a network designed for safety-critical applications in avionics systems. The flow analysis is mandatory for the certification. The end-to-end (ETE) delay upper bound has to be computed and hence guaranteed. Different approaches have been proposed to compute ETE delay upper bound such as the Network Calculus and the trajectory method. Both methods are deemed to introduce some pessimism in the computations. We only focus next on the trajectory approach. This paper shows that the original trajectory approach is flawed and can compute an optimistic ETE. We present a counter-example and discuss the sources of optimism that must be considered to revise the trajectory method.