HTML Editing Error

This is a wired situation, I recently installed a new WP.MU site, and I try to add a HTML code when editing the contact page. First, I switch to HTML mode in page editing, then I copy the code and paste in the content, and update the page. Before I update the page, I have tried to see the visual mode to make sure the form is correct, it did correctly appear on the visual mode. But, after I updated the page, the form broke, and the HTML broke too.

I have tried several time with and without switch to the visual mode, or even disable the visual mode, but it still the problem.

However, other WP or WP.MU site that I created works fine, and do not have this wired problem at all. Don't know what is going on.