During the execution of the load tests protocol, the loader instance will set its _top_level_dir attribute, and this will persist if the loader is passed on to another load_tests method.  If that method does not specify a top_level_dir in a discovery call, the wrong top_level_dir may be used, and loading will fail.

You can reproduce this by having two test *packages* with a load_tests method that calls discover in their __init__.py, and specify both package names to the unittest CLI.