test units should be designed with rules of re-entrance in mind. Although it is unlikely that a given set of Test Units could be run simultaneously against the same device, it is possible that more than one of the same type of device (or devices which are tested by the same TU code) exists in the system. Since it may be desirable to run the Test Units concurrently as part of a system exerciser or a stress test for a specific subsystem, it is possible that the same TU code may be run in different threads under the same process. The use of static variables in this case could lead to data conflicts between the multiple instances of TU code execution.