org.apache.wicket.ajax.AjaxEventBehavior#onCheckEvent() will throw an exception if the event name starts with 'on', e.g. 'onclick'.
There are valid use cases where applications may use custom JS events with names which start with 'on', so Wicket should not prevent them.
During the milestone release org.apache.wicket.ajax.AjaxEventBehavior#onCheckEvent() will throw exception to really make developers fix the WARN message they have ignored in Wicket 7.x.
The method has 'protected' visibility, so it could be overridden by applications which need to use custom events with 'on' prefix in their name.