The problem is that when we reach the end of a region during card marking, we clear the cards between card and card_of (end). If card_of (end) is equal to card, we end up clearing nothing at all.

A similar problem occurs when cards happen to be set between end and the end of the reserved memory of the region - these cards would never be cleared.

Both issues don't affect correctness, but they do affect performance for regions with very few cross generation pointers.