It's one class of errors that doesn't typically happen with a static compiled language, people like to feel 'safe' about the dullest things but typos can be found pretty easily by running the code you just wrote in a REPL or at least structuring your application so that you can quickly get it to the same state to test live... But honestly you can just repeat the typos in your tests, or make new ones, in any case for me at least it's a class of error that comes up very rarely regardless of whether I'm using mitigations like auto-complete. And lastly I'd think if you were unit testing "properly", aiming for good coverage, which unit testing advocates will say you must do regardless of a static or dynamic language, your unit tests should catch various typos as a side-benefit. Testing things like int in -> int out or "reading this function it calls these methods on foo, here's a mock foo and we're just going to make sure all of them get called as expected and I didn't misread/mistype either the code or the test code" seems insane.