When you edit a page that contains a html form (eg. a contact form), Tiny MCE cuts the content after the first </textarea> tag. It always appears when you edit a page (not tested with a post) and you use Tiny MCE by default as the first editor. If the HTML is the first editor displayed, there is no such bug.
The rest of the content is displayed just below the edit area, but lays outside the edit area.
See the picture here:
<http://demo21.ovh.com/5bfed4cf73c151b9fc0f6b5e71f4bc87P/?see=bug-tiny-mce-and-form.png>
You see, above the "send it" button? Then, everything below this separation is lost, it is not in the edit area anymore (no "send it" button, no privacy statements, etc.), even if I switch to the HTML area. This cutting always happens after the first </textarea> tag when Tiny MCE is activated first.