We are about to talk about client validation soon, but if you already know about it, read on; if not, wait until my next post and come back to this one :-).  I’m writing this now because I just found it.  If you use client validation, it’s important to ensure you enable client validation before the form gets rendered.  The extensions append a unique form ID to the form element, and thus, if you don’t enable validation, this imporant required form id doesn’t get set (is null) and raises a client-side error.  So be careful to the ordering, it should be in this manner:

<% Html.EnableClientValidation(); %>

<% Html.BeginForm(); %>

And everything will be alright 🙂