[jacobolus]

The biggest problem is when users of software, programmers of software, and the software code itself have 3 different incompatible theories of how it works.

Sometimes it gets worse still: you can have different theories according to (a) scientists doing basic research into physics or human perception/cognition, (b) computer science researchers inventing publishable papers/demos, (c) product managers or others making executive product decisions about what to implement, (d) low-level programmers doing the implementation, (e) user interface designers, (f) instructors and documentation authors, (h) marketers, (h) users of the software, and finally (i) the code itself.

Unless a critical proportion of the people in various stages of the process have a reasonable cross-disciplinary understanding and effective communication skills, models tend to diverge and software and its use go to shit.

[taneq]

This is why dogfooding is so important - you're updating the programmers' model to align with the users' model, reducing the total problem space (and thus the available avenues to get it wrong) by many degrees of freedom.

[azernik]

This is why during software design the first thing I talk about is not the UX flow or the software architecture, but the user's mental model (or the mental model we want to give them).