Too Long; Didn't Read
It can happen that the result of a unit test changes over multiple executions of a test suite without any change in the code. Such a test is named a flaky test. It is not dangerous per se but reduces the confidence a developer can give to his test suite, diminishing the benefits of the latter. In a project where continuous integration was broken during the 5 first days of each month, it led to the tests being faulty at the beginning of the month. One way of fixing such issues would be to sandbox the execution of the tests to control the current date.
Share Your Thoughts