Wow,
That is really weird.
Seems like a bug in the parsing of the content of the POST. I wonder if it exceeded a limit of the textarea. That is a definite limitation of editing long documents in a browser.
On openacs.org you should be able to upload a new revision of a document. Apparently I did not add the feature to upload content for an index. page so that won't work in this case. Normally I think that item should have a page and not a subtopic.