At the moment, if you accidentally tick the "Allow in mastery paths" tick box in Page | Edit, and you have Mastery Paths switched on but are not actually using them in a course, the following things happen:
1. The Page is invisible/inaccessible to students regardless of whether it, and its containing Module, are published;
2. The Page appears in the Assignments list (see: Why is there a page in my Assignment Index? )
This is very unintuitive behaviour - the current wording suggests that ticking the box will permit extra functionality, not covertly interfere with normal behaviour.