I fixed this by making the auth fields into ThreadLocal variables and ran it a few hundred times to make sure that the issue was fixed. Marking as fixed.

Yep, this is a thread safety issue in AbstractSpringJerseyRestTestCase. We are reusing the credentials stored in the super class of the test in multiple threads. I'll fix this by making the credentials ThreadLocal variables.

You also get this exception sometimes:

This is strange because the REST service is just asking the Spring Security context for role membership for the current user. There must be some thread-safety issue here because the other 20 or so requests seem to work perfectly fine and indeed, the test usually executes successfully.

The failure appears to be:

and then: