When you change the default format for wiki pages, the default-installed pages (i.e. Help, Front Page, User's Guide) stop working. It might be wise to (at least in the case of Help, perhaps in the case of Users's Guide) specify the format in the metadata blocks.
I was going to submit a patch for this but I'm not sure what to do with the User's Guide, since it's copied directly from the README which is presumably also used for other things.
Include format metadata in default installed pages.
This allows them to continue working when the user changes the
default page format.