WebKitTestRunner removes user content before starting a new test. But this means that user content scripts run in about:blank that is loaded between tests, and can produce output. They can even install timers that will run when WKTR thinks that it's already running a new test.
Steps to reproduce: run these tests in order, in a single process:
Created attachment 261613 [details]
Comment on attachment 261613 [details]
Clearing flags on attachment: 261613
Committed r190035: <http://trac.webkit.org/changeset/190035>
All reviewed patches have been landed. Closing bug.