[_bxg1]

The problem is when it changes in ways that are unpredictable. All UIs change in certain ways as we interact with them, but users are constantly trying to create their own internal model of the interface's behavior so they know how to do different tasks repeatedly. We learn rules about how dropdown menus work, for example, or which nav links go to which screens (and how to get to other screens from there).

It's problematic when that underlying model is too obscured (not communicated well enough) through visual cues. It would be actively consternating to constantly shift it underneath someone who's trying to wrap their head around it.