Unlike testing in a laboratory or test bed situation, the testing of production systems requires precautions to avoid side effects that might damage or disturb the system, its environment, or its users. This paper outlines safety precautions to be taken when testing production systems. Specifically we discuss precautions for penetration testing aiming at identifying security vulnerabilities. We generalize and document experience we gained as penetration testers, describing how the risks of testing can be mitigated through selection of test cases and techniques, partial isolation of subsystems and organizational measures. Though some of the precautions are specific to security testing, our experience might be helpful to anyone testing production systems.