I'm one of the "Potterheads" who saw Fantastic Beasts: The Crimes of Grindelwald over the weekend. A lot happened and I found it very confusing. Let's focus on one of the minor details.
It's the strict but fair Head of Gryffindor House, deputy and future headmaster of Hogwarts School of Witchcraft & Wizardry, winner of the Transfiguration Today Most Promising Newcomer award, the silver tabby cat, Minerva McGonagall! Beloved by millions of muggles for, among other things, her excellence in ballroom dance.
Harry Potter and the Goblet of Fire - Ron and McGonagall's dance (HD) www.youtube.com
Her inclusion in Rowling's new Fantastic Beasts movie could be considered a welcome sighting for a fan-favorite. It's also problematic, and not just because Fiona Glascott is following in the footsteps of Maggie Smith.
Minerva McGonagall's origin, as revealed by Rowling's writings on her Pottermore website, is fascinating. She was born in the Highlands of Scotland to Robert (a Presbyterian minister) and Isobel (a witch) McGonagall. Isobel hid the secret of her magical identity from Robert for years. She had become estranged from her family after eloping with a muggle against their wishes. It wasn't until the uncontrolled prowess of little Minerva became too much to hide that Isobel came clean to her husband. Robert, of whom Minerva was known to take after, was drawn into the binding conspiracy of the magical community. Minerva was quick and bright from a young age; enough so that she felt the irreconcilable differences and broken trust that darkened her family. Despite the difficult nature of their world and their union, Isobel and Robert together raised their daughter to become the greatest witch of her time.
That's the Spark Notes version, and it doesn't do enough justice to pass Professor McGonagall's standard. There's also a controversial Gryffindor Quidditch career, a broken-hearted ex-fiancé, and her own marriage, cut tragically short by the potent poison of the Venomous Tentacula. Not to mention her close friendship with Albus Dumbledore. Read in J.K. Rowling's full detail on the subject, here.
This sounds great! So, where's the problem?
Minerva McGonagall has one scene! Two lines! We briefly get to see her face. Her inclusion is a cheap cameo that got a good chuckle out of the audience. To most people, this complaint reads like an overly attached fan criticizing a small detail in a movie that doesn't deserve this much attention. But, Professor McGonagall's appearance throws a wrench in her story.
The Crimes of Grindelwald's primary action takes place around the Wizarding World in 1927, during the rise of the titular villain. The moment we get a glimpse of McGonagall is in an interesting Hogwarts flashback into the childhoods of Newt Scamander and Leta Lestrange. Albus Dumbledore is teaching Defense Against the Dark Arts and Minerva McGonagall is scolding third-years.
This seems harmless. But it's not!
Rowling is throwing out a fun scene in her fifth book, Harry Potter and the Order of the Pheonix. It's when all-time supervillain and Hogwarts High Inquisitor, Dolores Umbridge, is evaluating Professor McGonagall.
She later informs Umbridge she's been teaching at Hogwarts for "thirty-nine years, this December." Readers have pieced together the timeline of events in the Wizarding World using a handful of details across the series. The most important of which is the death of Harry's parents, James and Lily Potter, in 1981. This logic, combined with Rowling's further writings, concludes that Minerva was born in 1935. In other words, it makes absolutely no sense that she's in this movie.
It's an annoying habit of all fandoms to nitpick and expose the contradictions in a universe's canon. It's all made up, anyway. Not everyone is J.R.R. Tolkien. But, it's a worrying trend for Rowling's creation. She's approaching the point where we can start saying she's Terminatored the shit out of this one. And no one wants to see that happen.
McGonagall's ill-advised cameo is at the bottom of the list in terms of weird decisions Rowling has made with the rules in her Wizarding World in The Crimes of Grindelwald. There's plenty of confusing ancestry. Nagini is in there for some reason. At least we have "Hot Dumbledore." There's also plenty to hold onto for long-term fans of the series. Eddie Redmayne continues to give the best performances of any actor in the ten-movie series. He communicates Newt Scamander's anxieties with the same nuance that won him an Oscar for playing Stephen Hawking. It's the fact that this sequel doesn't lean on him enough that leaves it short of Fantastic Beasts and Where to Find Them. I will, however, continue enjoying these movies even if they continue to be two-hour-and-fifteen-minute set-ups for a five-film franchise based on a fictional encyclopedia. Sometimes we really don't choose who or what we love.
 "Known to successive generations of students as 'Professor McGonagall', Minerva – always something of a feminist – announced that she would be keeping her own name upon marriage. Traditionalists sniffed – why was Minerva refusing to accept a pure-blood name, and keeping that of her Muggle father?" -J.K. Rowling (Pottermore)
 This comes from Deathly Hallows, where Harry and Hermione visit his family's grave.
 [furiously pretending The Cursed Child doesn't exist.]
 This is pretty disappointing. Fantastic Beasts and Where to Find Them is still my favorite of the Wizarding World's film entries, and it's mostly because of Newt.