什么会导致MVC4控制器只返回状态代码而不是部分视图？

This is sort of an odd question but then I have a very odd situation. On my development server (IIS & IIS Express) I make a ajax request and return a form. The user then posts the form back via an ajax post and if there are any validation errors the controller sends back the partial view with a response code of 400. In my ajax method I intercept any 400 errors and redisplay the form with the validation errors. This is sort of an odd question but then I have