The biggest one would be Bianca's party in books 3, where we see Cowl for the first time (as a hooded figure barely mentioned), we see the athame (that is now obviously important/powerful, though we still don't know why).

Ferrovax the dragon, who seems very likely to make an appearance again at some point, was there as well.

Additionally, the various revelations involving Thomas were hinted at and spread out through a few books.

There have also been tons of other small references and hints of things to come- who betrayed the wizards at Archangel? Why was Winter interested in Molly from as early as book 3? Who provided knowledge of the Hexenwolf belts and the heart-stopping curse from books 2 and 1?



My thinking was that Calvin and Hobbes and Little Chicago were examples like that- minor things, hints that will be referred to if there's a time travelling book. (And considering there's an entire law devoted to banning time travel, it does seem likely. We've already had necromancy, mind control, stuff with Outsiders...).

For example, Harry's back in time at the time of Small Favor for whatever reason. He needs to swipe a book, so he sneaks into his basement while his past self is out. Maybe he gets nostalgic and reads an old Calvin and Hobbes when suddenly, he hears a door opening! Cursing, he shoves the Calvin and Hobbes into the first box he sees and gets the hell out of there before he's seen.


Little Chicago (in Proven Guilty, btw, just looked it up)could be similar- Harry needs to use Little Chicago for something, but remembers that it had a problem. Now that he's older and wiser, he fixes it, uses LC, and goes on his way.