As to front end testing being difficult, there is a certain class of combinatorics that is difficult to test exhaustively. However, it's not hard to test that each validation error displays properly, and that multiple validation errors display properly. That's usually enough to convince a sane person that any combination of errors are going to display properly. (Assuming a reasonable developer who's planned for the errors area to grow with the list of errors)

Are there really UIs that allow users to do things never imagined by the developer? I'm having a hard time thinking of one. Photoshop has a very complex UI, but each action generates transform that is layered onto the image. Automator lets you do things never imagined by the developer, but in reality it's just a series of pipes between actions. The UI lets to pick actions and arrange pipes.

I'd argue (though perhaps be wrong) that getting beyond what's been thought of by the developer requires code or at least a domain specific language, not a UI.