Why after a form post back, I get 404?

Question

I am creating a contact form page. Perhaps I am doing it wrong? I have something like

<?php
// even when I remove this validation block it fails
if (isset($_POST['name'])) {
    // do validation ... 
}

get_header(); 
?>
...
<form id="frmContact" action="<?php the_permalink() ?>" method="post">
    ... 
</form>
... 

I used the same logic in another page and it works ... the URL is correct and if I refresh the page, it works. It just gives 404 on post back