When you have Visual Studio 2008 and ASP.NET 3.5 Extensions installed you may run in following error.
BC30560: 'ListView' is ambiguous in the namespace 'System.Web.UI.WebControls'.
In my case, this happens when i drag an drop a Listview Control from toolbar. Then Visual Studio adds following line in web.config
<
add assembly="System.Web.Extensions, Version=3.6.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/></assemblies> Remove it - and it should work again.
In a second case, same action, added following in ASPX page
<%
@ Register assembly="System.Web.Extensions, Version=3.6.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" namespace="System.Web.UI.WebControls" tagprefix="asp" %> Remove it also
seems to be a bug from 3.5 extensions.