Hi Nat,
I'm experiencing the very same problem. It seems to have been when I've been in the process of creating CMS fields, but even when I've reverted to standard Page Classes it still seems to bug out.
For a temporary fix, I have replaced the TinyMCE field as a HTMLField so I can still load content in the meantime. To do this, edit your /mysite/code/Page.php file and add the following function (or edit your existing one):
// === construct CMS fields === //
function getCMSFields() {
// get existing fields
$fields = parent::getCMSFields();
// remove Content CMS field
$fields->removeFieldFromTab('Root.Main', 'Content');
// add Content CMS field as textareafield
$fields->addFieldToTab('Root.Main', new TextareaField('Content', 'Content', 20));
return $fields;
}