When I worked at Mozilla, we had an issue where the unittests will always past, except on very few occasions. The message was not entirely obvious what the problem was (from as far as I remember) so we ignored it for a bit as it didn't seem to impede any work.
We eventually realized that the unittest will only fail after work hours, at which usually no one is around to start tests. It turns out the test will fail during 6PM - midnight because of an UTC issue.
We eventually realized that the unittest will only fail after work hours, at which usually no one is around to start tests. It turns out the test will fail during 6PM - midnight because of an UTC issue.