I came across a postback issue while using the CascadingDropDown AJAX control. I had a web page with 3 DropDownList controls. The data for these controls was populated from the CascadingDropDown which would bind the data at runtime. The issue with this is that when data is being bound at runtime, it cannot be validated during postback unless each value is registered with the page. In most cases, this wouldn't make sense. So, we have to fall back to turning off EventValidation at an application level in the web.config (attribute is found on the pages element) or at the page level (attribute is added to the page directive). It's best to turn off the EventValidation at the page level. There is no way to turn this off at the control level unless you are building your own control that does not contain the SupportsEventValidation attribute.