Could be a 404 bug, but maybe it's by design?

[arretx]

When a page has a minimum membership level required and the user is NOT logged in or does NOT meet the membership level, they should be redirected to the membership options page that has been set.

That works.

UNLESS, there's also a custom capability set for that page too, in which case I get a 404 page.

The only solution I've found is to use simple scripting to show conditional content in the body, which means I'm duplicating a lot of simple conditionals.