Please provide a URL. This is not something that can be debugged in the abstract.
Agree with Sanford. Impossible to know without seeing.
The one time I had this happen to me, though, it was because I had another form on the same page (a blog subscription email only form) and the form calls were not unique.
just an idea... if you have a Landing Page Template where you can disable sections, make sure there's no Form pulled into those sections before "disabling" them.
That disabling mechanism is often achieved with a simple display:none inside the HTML, which might not work if theres a form inside.