Error A Page Can Have only one Server Side Form tag

Question

I am Getting Error A Page Can Have only one Server Side Form tag.But there are no multiple server side form tags on page neither any master page has been applied to the page.It is usually coming while i am exporting grid to pdf.I am able to export.But Sometimes this kind of exception comes.Do not know why please do let me know about the cause of this error.I am stuck on it.