For the term to be meaningful, 'bug' needs to be more tightly defined - a discrepancy between the specification (both as stated and intended[1]) and the actual behaviour of the code is a bug. Now, if the wrong behaviour is actually beneficial, then there's nothing wrong with changing the spec, that's basically the central idea in agile - but the idea that if given a spec to make the blue button green, you make the button green - and if users then react negatively to that, then that doesn't mean you shipped a bug in any meaningful sense. It also doesn't mean you shouldn't change it back, quickly, but the developer did not ship a ship.

1: Just because the specification doesn't specifically state "shouldn't crash if the user inputs a negative number", it's understood that the program shouldn't crash given any user input.