When we use the AJAX Control Toolkit ModalPopupExtender, sometimes we may get below error

Sys.Extended is undefined
$create(Sys.Extended.UI.ModalPopupBehavior, {"BackgroundCssClass":...

After few hours of investigation, I realized that keyword I’m using it for scriptmanager.
If you are using .net 4.0 framework, then change it from

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager>

To

<asp:scriptmanager  ID="ToolkitScriptManager1" runat="server"></asp:scriptmanager >

And vice-versa for 3.5 and below version.