I encountered a strange problem with the Visual Editor on one of our sites. Whenever the user clicked either the "Paste As Plain Text" or "Paste from Word" buttons they received a "Well that is somewhat embarrassing" 404 message.
I eventually tracked it down to the attempt to bring in /wp-includes/js/tinymce/plugins/paste/pastetext file. The pastetext.htm file is there.
The cure was to copy the /wp-includes/js/tinymce directory from a working site. I have no idea when this started or how it got broken, but numerous WordPress updates had not fixed the problem. I suspect this might be related to http://wordpress.org/support/topic/tinymce-error-missing-js-files-breaks-on-every-upgrade?replies=19