Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack.

• For Response.End, call theHttpContext.Current.ApplicationInstance.CompleteRequest method instead ofResponse.End to bypass the code execution to the Application_EndRequest event.
• For Response.Redirect, use an overload, Response.Redirect(String url, bool endResponse) that passes false for the endResponse parameter to suppress the internal call to Response.End. For example:
  Response.Redirect ("nextpage.aspx", false);
• If you use this workaround, the code that follows Response.Redirect is executed.
• For Server.Transfer, use the Server.Execute method instead.

http://support.microsoft.com/kb/312629/EN-US/