I've encountered this significant bug which is also reported in this support forum thread:
The problem is that if you try to save a post or page with 'too much' content, the content does not appear in the page. Disabling the content filter allows it to be displayed.
Using visual mode the problem is even worse because visual mode uses the content filter. If you save your content in visual mode it simply disappears. This would result in data loss if it were not for the revisions storage.
I encountered the problem using WP Version 2.6.1