The Machzor is the prayer book we use on holidays, and there are separate ones for Rosh Hashanah, Yom Kippur, and each of the three pilgrimage festivals. Each has various insertions for the holiday.
As a result, there will be additions that aren't seen more than once or twice a year, which is enough to throw anyone off balance.
So, if you're feeling lost or confused, recognize you're not alone.