You'd have to have the lamp-like eyes of Filch's cat Mrs Norris to spot this one: Newt Scamander actually appears in Harry Potter and the Prisoner of Azkaban, and not just as a name on a textbook.
Yes, that's right: when Harry Potter film spin-off Fantastic Beasts and Where to Find Them was just a twinkle in JK Rowling's eye, back in 2004, the magical creatures expert still got a movie debut - on the Marauder's Map.
According to Pottermore, Scamander was born in 1897, and by the time the third novel takes place it's 1994, putting the textbook author at a grand old age of... 97. Old, but just about feasible that he'd be alive and visiting Hogwarts.
The question is - why?
We do know from Fantastic Beasts that he was good buddies with the young Albus Dumbledore way before the headmaster grew his flowing white beard – though it wasn't enough to stop Scamander getting kicked out of Hogwarts for an as-yet-unknown reason. So could they just be friends?
Alternatively, he could be helping out with one of the many magical creatures hidden within Hogwarts' halls and roaming its grounds. Whomping willows, centaurs, unicorns, a phoenix – who better to advise than ancient wizard Newt Scamander? Particularly when you remember that Azkaban was the first time we met the proud Hippogriff Buckbeak...
Perhaps Rowling will enlighten us in one of the five Fantastic Beasts movies, which will span 19 years. That's plenty of time to clue us in to what exactly Scamander was doing on the Marauder's Map all those years later.
Harry Potter and the Prisoner of Azkaban is on ITV on Boxing Day at 4.10pm