User:KOLI@fawiki informed me that after saving any edit, he get redirected to Special:BadTitle instead of the saved page. The edit is saved and can be found in Special:Contributions/History. This doesn't happen using VE (or editing through gadgets for obvious reasons).
This is the picture he sent me:
(The translation of the mediawiki message is "Title of the requested page has bad character: Foo")
I can't reproduce it but since it happens for all pages when the user edits, it seems important to me.