It's strangely satisfying when the "this will probably never happen" test case finds a problem during development.

I had tests for deleting that were like

• create item a
• create item b
• delete item a via the code under test
• assert item a is gone
• assert item b is still there

I thought maybe the whole bit with item b was excessive, but sure enough one day I accidentally fucked something up and deleted all the items, and the test pointed it out before the bad code left my local machine.