When a RadSlider is in Lightweight render mode and it is configured to be vertically oriented, its decrease and increase handles are positioned outside of the container of the control. As a result they could appear in elements that are located above and below the slider.

Currently this can be avoided if you add margin to the slider:

    <style type="text/css">
        .rslVerticalSlider {
            margin: 2em 0;
        }
    </style>

    <telerik:RadSlider ID="RadSlider1" CssClass="rslVerticalSlider" runat="server"
             Orientation="Vertical" RenderMode="Lightweight">
    </telerik:RadSlider>