WTF code, defects, and the principle of least astonishment

We have recently introduced two defects, while trying to improve old, obscure code. Of course we are to blame for not being more paranoid and not researching more why the code was there. But if the code was clear to start with, if it followed the least astonishment principle, the errors wouldn’t have happened. In both cases the original code provoked the “WTF?!” reaction in the reader.