1. A non-transitory machine-readable storage medium comprising instructions executable by a processing resource to: instruct an external service virtualization system to simulate a fault of an external service in a simulation of the external service, wherein the simulation of the external service is accessed by an application during testing of the application; generate a testing application fingerprint representing a response of the application to the simulation of the fault of the external service during the testing of the application; and associate the testing application fingerprint with the external service and the fault in a repository storing the testing application fingerprint.