Yeah, but it isn't usually very difficult to write a test correctly, unit tests especially.
If you can't write a test to validate the behaviour that you know your application needs to exhibit, then you probably can't write the code to create that behaviour in the first place. Or, in a less binary sense, if you would write a test which isn't "right", you're probably just as likely to have written code that isn't "right".
At least in the case with the test, you write the test and the code, and when the test fails (or, doesn't fail when it should) you're tipped off to something being funky.
I'm sure you could end up writing a test that's bad in just the right way to end up doing more harm than good, but I do think that's the exception(heh).