The case study describes rotor crack detection using vibration measurements, documented on real case of Induction Motor driven Compressor Unit. The study highlights the complexity of rotor crack diagnostics as the primary problem as it can be often masked by other existing machine malfunctions. In this case indication of the possible rotor crack, led to the Compressor bundle removal and rotor inspection. The rotor problem was confirmed, but not due to rotor crack but luckily only due to rotor thrust collar fit found to be worn and locating pin to be bent. Thrust collar had become eccentric (radial run out measured) causing a mass unbalance on the rotor in one direction, with similar symptoms as rotor crack.