This is a great example of the value of test plans. This is basically technology to reconstruct missing test plans via the code. But of course someone already knew about the subtle "wall kick" feature since she or he wrote that code. It shouldn't be this hard, with some effective communication.

And actually, especially in games, test plans are still poorly communicated. In the old days, it was awful -- you would have the publisher doing all the QA, and barely speaking to the development team apart from bug reports. QA still often doesn't get involved until the last half of the project, before which time nobody has been thinking much about testing.

As studios improve their production, this is getting better. As a programmer, I've had more collaboration with QA as I work, at its best including having our QA liaison talk out a test plan with me while I'm working the feature. With enough communication, hopefully this kind of detective work to figure out what to test can be avoided.