…trunk and would break the test runner due to different code origins if mixed with unittest symbols.
Most of the time this doesn't cause any problems, unless you have angle brackets in your exception information. Such as, say, in tests involving markup.
The XML test runner was previously doing a bunch of extra work instantiating a test database in a manner tied very closely to the single-database model from Django <1.2. Django 1.2 and above provides a test runner class which can simply be subclassed. There's some slightly hacky code to switch between the two.