For example, using this markup:

<telerik:RadEditor ID="RadEditor1" runat="server" ToolbarMode="RibbonBar">
</telerik:RadEditor>

and this Page_Init code:

protected void Page_Init(object sender, EventArgs e)
{
    RadEditor1.RenderMode = RenderMode.Lightweight;
}

Causes icons to either miss or their position is incorrect.