See http://bugs.python.org/issue20954. I'm not sure how (and if) this can be handled gracefully in tox.
My first idea was setting PYTHONHASHSEED=random per default (I already prepared a PR for that). But then I noticed that this will change the current behavior completely: with PYTHONHASHSEED=random every env will get its own hash seed and you won't get a useful output on run (because there will be
PYTHONHASHSEED='random' in the log. So you won't be able to fully reproduce test runs which was the main intention of pull request #79 I guess.
A less intrusive option would be (on Windows): if a test run fails, scan the output for a
MemoryError in subprocess. If it matches point to this bug and give the suggestion to run tox with
--hashseed random. But I don't think how you feel in general about handling upstream bugs (please note, this Python bug will never be fixed in 2.6 and 3.3).