What is the "action" of your form set to?
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html
Problem resolved.
I remembered seeing this a long time ago. If you have a page that has a lot of JavaScript (in this case to validate and submit a form to our Eloqua back-end marketing automation system), and when you are using Firefox (vs. IE), MODx doesn't like saving a page if you edit in WYSIWYG mode.
Once I toggled the editor into HTML only mode, the page saved fine.
An inconvenience but page saves fine now and form works fine.