This kind of error can happen when you are dynamically adding controls to the page. You need to make sure the same controls are created during the init event during the postback request.
Also, you must keep in mind that PageLoad event always happens before button click events so if you are initializing control values from page load you often need to wrap it in if(!IsPostBack) so that you don't set them again during postback.
More help from google.
Hope it helps,