Error: The target “ControlID” for the callback could not be found or did not implement ICallbackEventHandler.
Scenario 1: You try to dynamically add web controls to a ContentPlaceholder or Panel and on a postback of the page caused by triggering an event from the web control, you get the error.
Cause of Scenario 1: When the event is fired, the generated web control that fired the event is no longer on the page.
Resolution of Scenario 1: Make sure you add the dynamically generated web controls on every page load. Ex: Generate controls on !Page.IsPostBack and Page.IsPostBack cases.