You may be getting this message in Page Editor after clicking any of the top bar buttons (Edit, Design, Log off).In my case it is caused by the System.Web.UI.WebControls.ImageButton control's PostBackUrl property which I am assigning in code behind.It appears that any control with that property assigned and used such as asp:LinkButton, asp:Button cause this problem on SR-1 installations and presumably earlier too.To workaround it, I added a check for IsPageEditor in my control:if (!Sitecore.Context.PageMode.IsPageEditor){ imageButton.PostBackUrl ="/mypage.aspx";
} Works as temp workarond, the tech support will be investigating further.