MVC Error: 0x800a1391 - Microsoft JScript runtime error: 'Sys' is undefined

If you get the error:

MVC 0x800a1391 - Microsoft JScript runtime error: 'Sys' is undefined

Then first check to make sure you have included the correct MVC javascript files:

C#:

<script src="<%= Url.Content("~/Scripts/MicrosoftAjax.debug.js") %>" 
    type="text/javascript"></script>  
<script src="<%= Url.Content("~/Scripts/MicrosoftMvcAjax.debug.js") %>" 
    type="text/javascript"></script>

VB.Net:

<script src="@Url.Content("~/Scripts/MicrosoftAjax.js")" type="text/javascript"></script> 
<script src="@Url.Content("~/Scripts/MicrosoftMvcAjax.js")" type="text/javascript"></script>