By deliberately allowing different tests to run in the same process, you can easily find hidden assumptions about state.

One good way to shake out dependencies is to run the tests in semi-random order. "Semi" in the case means at least temporarily reproduceable, to allow for debugging. A switch to run the tests in reverse order is a trivially easy way to flush out some problems. Seeding the random number generator with the epoch date (but not the time) before randomizing test order allows for same-day debugging.