Handling validation errors on Ajax calls in Asp.net MVC

If you're developing rich web clients using Asp.net MVC on the back-end, you've probably come across this functionality that can be described with these conditions:

1. client side data (may be a form),
2. ajax posting of this data (form),
3. controller action has strong type parameters,
4. controller action processes data and returns anything but a full view (since it was an Ajax call)

Familiar? Then you've come across this problem: »What should be done when validation fails?«
Seems fairly straight forward, right? Well not so fast my fellow developer friend...