I THINK I just might have found a bug.
I took a working page from my site and use the coding to create my own contact form on my site.
But every time I submit the form (the form post to the same page so action="") I ended up on the 404 page, even the url in the address bar is for the correct page.
Don't ask me how I got to the problem (played around a bit), but I realized the reason is one input field are named "name (name="name"). This work perfect on my usual php contact forms.
As you can see this contact form ( http://www.lenocin.com/index.php/contact/ - it's messy there....still working on my theme) just works fine now.
So I renamed the textbox to "_name" and all work as it should