Control 'GridView1' of type 'GridView' must be placed inside a form tag with runat=server

To know about creating a Grid view with Paging and Exporting the view to Word and Excel documents read this article.
The following is the screen shot which you get due to this error:

This error occurs whenever you are trying to export the grid view data to excel or word. This occurs due to the reason that the compiler thinks that the control is not added in the form. Hence to resolve this issue, add the overriding function in the code

public override void VerifyRenderingInServerForm(Control control)
{
  /* Verifies that the control is rendered */
}

This code will fix this issue.