REPRO

    Run the snippet below
    Inspect the DOM (at the beginning of the form you will see the .rtbSlide element)
    Click the toolbar button to invoke an ajax request

actual: the .rtbSlide element is multiplied with every request

expected: the toolbar is fully disposed

        <asp:UpdatePanel runat="server">
            <ContentTemplate>
                <telerik:RadToolBar RenderMode="Lightweight" ID="RadToolBar1" runat="server"
                    AutoPostBack="true">
                    <Items>
                        <telerik:RadToolBarButton Text="postback to test">
                        </telerik:RadToolBarButton>
                    </Items>
                </telerik:RadToolBar>
            </ContentTemplate>
        </asp:UpdatePanel>


WORKAROUND

        <script>
            var oldDispose = Telerik.Web.UI.RadToolBar.prototype.dispose;
            Telerik.Web.UI.RadToolBar.prototype.dispose = function()
            {
                if (this._moreButtonDropDown && typeof this._moreButtonDropDown.dispose == 'function') {
                    this._moreButtonDropDown.dispose();
                }
                oldDispose.call(this);
            }
        </script>