I started building a site with SharePoint 2013 Foundation. After the purchase came through, I upgraded to SharePoint 2013 Server. I had enabled anonymous access to SitePages, and that continues to work after the upgrade, but I can't seem to access the new Pages library (added by Server) anonymously. Both libraries inherit their permissions from the site, and they both show that Anonymous Users have the View Items permission.
I don't really think that this has anything to do with it, but I created the pages in SitePages from the ground up in SharePoint Designer. The pages in Pages were created in SharePoint proper from layouts.