In my experience an even more pernicious problem is unit tests that are unreliable or too expensive (in time especially). Devs too easily come to allow unit test suites that routinely take reruns in order to get to 100% passing. Similarly, when build verification tests or CI tests get too bloated it can be hard to pick where to draw the line.
As a real life example of how this can happen, PHP 5.3.something release had a serious bug with MD5 hashes essentially not working. (cf. https://bugs.php.net/bug.php?id=55439 http://www.php.net/archive/2011.php#id2011-08-22-1 ). Apparently there was a unit test for it, but there was ~200 failing unit tests, so they ignored it.