I admit I still catch myself accidentally hitting the browser's back button on a multi-page Ajax form without its own back button. Or having to fill out a form again because, instead of opening a modal or a new tab, the current page changes (e.g., TOS). I've come across countless UX developers who don't consider these subtle details.

And this is only a small part of nuances that a typical user faces. I don't care how beautiful or fancy your UX is. Familiarity is king in design, and if you stray too much away from the current experience, I won't hesitate to say that my toilet has better UX.