Factual error: The opening scene shows Donna graduating from New College in 1979. That is not possible, women were only first admitted in 1979.
Factual error: Towards the end of the movie when Ally is in her hallway looking at Jack's old concert tour posters hanging on the wall, the poster closest to the camera says "Washington State Fair" and a 2011 date. Unfortunately, that fair did not get that name until 2013. Before that, it was called "The Puyallup Fair" (officially named the "Western Washington Fair"). Locals still call it "The Puyallup Fair" to this day.
Other mistake: The snow globe from the original is in the loft in the new film, but in the original film, as Mary was leaving the Banks house while they flew a kite, she packed it in to her carpet bag.
Continuity mistake: When Buster is shot, his hat with the hole in it disappears.
Character mistake: Dumplin' is looking through her aunt's belongings, and finds an old application form for the Miss Teen Bluebonnet pageant. The application has an August 1993 signature but marks Lucy's DOB as 5/5/79 with a stated age of 16: it does not add up, as she'd have been 14 instead. Granted, it's an application she never actually turned in and not an official document, but this does not justify the mistake in the context of the movie (not to mention that had she just been too young to participate, the point of the movie would be moot).