ASP.NET MVC - HttpException visible in View Source in browser

Question

I developed an asp.net mvc application. In one of my forms, I'm getting a following exception:

A public action method 'UpdateBasket' could not be found on controller 'App.Controllers.WebShopController'

But the funny thing is, that form submit works (even in debug mode) and finds an ActionMethod UpdateBasket and returns View. The problem is that because of that html generated error w3c validators and google crawlers can't access the site (I think so, correct me if I'm wrong).

Has anybody got any idea why this error occurs? Thank you.