Hi staron,
I am not sure what exact event you mean by "window opened event." - in case you mean the OnClientShow event, yes, the described behavior is expected in some cases, when the content page needs more time to load. In case you need to ensure that the content page is already loaded, e.g in order to manipulate it, you should use the OnClientPageLoad event of the RadWindow which fires exactly when the IFRAME is loaded.
Sincerely yours,
Svetlina
the Telerik team