Hello Christopher,
Testing the logic of your code will catch the latter error and not the former. But the former isn't an error, if it has the same result.
IIRC dynamic language do gobs of TDD/unittests because they have no choice for just this reason. Any other approach and you have no clue what you'll get. Given a choice between an option that lets in static checking and one that forces the use of runtime testing/checking, I'll go for the static choice every time. (sorry if I'm getting OT)
