Matt Heusser quoted me on this at the Google Test Automation Conference. I've had this idea for some time, that many tests both manual and automated waste time explicitly testing for conditions that don't need tests.
I ran into one of those today. Or rather, my boss did. (He's not really a boss so much as he is an experienced colleague who has more conversations with management than me. Socialtext has very little hierarchy. )
We have a lot of automated tests that we created very quickly. Many of them have TODOs left over. I've been cleaning up the tests and implementing the TODOs.
One of the tests had a whole list of TODOs that had already been done, so I deleted the TODO list. My boss saw this and asked me why. In particular, one of the TODOs had been to create a record of a particular type, and my boss didn't see a test for this.
I pointed out that I had written an explicit test for sorting records of exactly that type, and three records of that type were created in the setup for the sorting test.
Had the record-creation failed, the tests for sorting would have failed. It's a nice economy of effort.