Large novels and series

Shared references and storylines that extend beyond one book.

Your series has become too big to remember reliably
Established facts across books are difficult to verify; the progress of one unresolved thread is a separate concern.
A storyline stretches across several books
The lifespan and development of one thread crosses books; this is more specific than general series fact checking.
Your novel has become too much to hold in your head
The burden spans several kinds of information, rather than one scene arrangement or cross-book fact being the sole obstacle.

Your series has become too big to remember reliably