Probes are employed to inject errors into code. In response to a function-entry trigger event, a probe writes a predefined test value to a return value register. The probe then cause function execution to be skipped such that the test value is returned in lieu of the value which would otherwise be returned by the function. Behavior after the error is injected may then be observed, data collected, etc. such that undesired behavior (e.g., crashes) can be identified and/or corrected. In an alternative embodiment, the probe which is triggered may write a test value to a given memory address.