Ironically, the one time where I noticed this bug was while testing a component, whose only use involved me emulating the behavior of mutable default arguments on a higher level. I still think it is a horrible system. My strong opinion on it might be because I spent hours looking at our c-extensions for the source, because the previous two 'language bugs' I ran into were both in our c-extensions, where we had a type that could not cast to it self, and someone thought it was a good idea to make ~x mean x-1.