When Confluence ancestor CONFANCESTORS table gets out of sync or corrupted. Page View restriction are not inherited to the child pages. This might be quite random, as in not every child pages are affected. IMHO, we should have
CONF-25188 implemented to help this out.
Please follow the resolution in this KB:
We've made some changes in 6.5.0 to detect if there is no parent ancestor record for a page. This change will block users from viewing pages with a missing parent ancestor record, and will log a warning for administrators.
The steps to resolve this issue is still to rebuild the ancestors table as described in this KB https://confluence.atlassian.com/x/8qr-Nw