I noticed changing the "Active Handler" on components experiencing this error from "JoomSEF default handler" to "JoomSEF basic rewriting" at least allows frontend access to the affected components.

Fatal error: Class 'JoomFishManager' not found in /home/xxxxx/public_html/administrator/components/com_sef/classes/seftools.php on line 359

When using Facebook Connect and multiple other components...

Never have installed JoomFish... Their is definitely a bug that is causing 404 errors to be "translated" incorrectly resulting in this error.

I concur.

Same!