There is an adage in web UX which goes 'Don't break the back button' which refers to the fact that the back button in a browser is such a consistent function that to create a situation where it's function is removed or changed leads to a bad user experience.
Within the context of human computer interaction in general there are other established conventions which are also extremely common. Such conventions include select something & Ctrl + C and Ctrl + N.
In some applications these conventions are broken and the user is required to learn application states or contexts before they are able to know when to use these functions or how to complete the same task from any given position, yet the functions could still perform an expected and controllable action while the application is in the aforementioned state.
Examples include Photoshop's transform state, during which copying what is selected does not work, or the PHP Storm removal of Ctrl + N to create new documents. In both cases the application would be capable of responding in an expected fashion to these commands.
Even in the most complex of applications is it ever acceptable to break such interaction conventions?