MB3 62a: Additional Maftir Laws (282:5-7); Why Only One Torah on Shabbos (283:1) Haftarah (284:1)

What if the only person who can do maftir already received an aliyah? What if a pasuk/letter was skipped in laining? Why don't we lain the Musaf as Maftir on Shabbos? The origins of reading the Haftarah.