The workaround is to avoid the other settings (e.g., Auto). In most cases Lightweight RenderMode will suffice, the only potential issue is loss of gradients, shadows and rounded corners in IE6-8.
Still not working with 2014.1.225.40 and <add key="Telerik.Web.UI.RenderMode" value="Auto" /> in web.config, IE10(Standard Mode). I get the error "RadTooltip with ID: ... has not defined RenderMode in getRenderer Type.registerNamespace("Telerik.Web.UI.ToolTip"); (function(){Telerik.Web.UI.ToolTip.RendererFactory={getRenderer:function(a){if(a.get_renderMode()==Telerik.Web.UI.RenderMode.Classic&&!a.get_enableAriaSupport()){return new Telerik.Web.UI.ToolTip.ClassicRenderer(a); }else{if(a.get_renderMode()==Telerik.Web.UI.RenderMode.Classic&&a.get_enableAriaSupport()){return new Telerik.Web.UI.ToolTip.ClassicRendererAria(a); }else{if(a.get_renderMode()==Telerik.Web.UI.RenderMode.Lite&&!a.get_enableAriaSupport()){return new Telerik.Web.UI.ToolTip.LightweightRenderer(a); }else{if(a.get_renderMode()==Telerik.Web.UI.RenderMode.Lite&&a.get_enableAriaSupport()){return new Telerik.Web.UI.ToolTip.LightweightRendererAria(a); }else{throw Error("RadTooltip with ID: "+a.get_id()+" has not defined RenderMode"); }}}}}};
The issue will be fixed in Q1 2014 SP1