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     Lately I've been noticing that some individual unit tests fail for 
me, even though the overall test suite succeeds.  So, for example:

$ /opt/stack/nova$ ./run_tests.sh
<snip>
OK (SKIP=5)

$ /opt/stack/nova$ ./run_tests.sh test_virt_drivers
AbstractDriverTestCase
     test_add_to_aggregate                                        ERROR  
0.02
     test_agent_update                                            ERROR  
0.02
     etc.

     And yet, if I scroll up and look at the earlier run (where 
everything passed) I see it running AbstractDriverTestCase with all green.

     What's happening here?  Does this mean that tests are coupled in 
some unhealthy way such that they can only be run in a particular order?

-Andrew