When Dunn is rescuing the cheerleaders from The Beast, a brunette cheerleader is hit with a table. When the other victims help her up, it's a different (now blonde) cheerleader.
Joseph mentions the "mutilations at the zoo," that is, the events seen in Fragmentado (2016), as taking place "three weeks ago." Also, several times it is mentioned that the crash of Eastrail 177 was 19 years ago. But at the end of Split a diner patron mentions Mr. Glass having been put away FIFTEEN years before (which was closer to accurate at the time of release). Since the train wreck and Elijah's capture were only a few days apart, both time frames cannot be true.
In the Guard Office, a large wall calendar shows June with 9 days "X"ed out, but a couple of days later, characters are sitting in a train station with a decorated Christmas tree in the middle of the station.
During the climatic final fight between the Beast, Glass, and David, the deciduous trees are in their Fall colors. Then just a short time later, Mrs. Price, Casey, and Joseph are in the train station and there is a large Christmas Tree. By the time December arrives in Philadelphia deciduous trees are bare.
During the battle inside the water tank, cracks are visible with air bubbles getting inside the tank. This is impossible since water is supposed to go out, pressure inside being higher than outside atmospheric pressure, as hydro-static pressure.
Elijah's scars are shown as a long line with several small lines crossing over it at regular intervals - these are meant to depict sutures from a surgery. However, an actual scar heals as a line bordered by dots, as only the places pierced by the needle would leave a scar. It is only suture material that crosses perpendicular to the original trauma line, which would leave no lasting mark.
Joseph shows David a news website reporting the attacks, but only the day and the month of the report is posted. All news websites have the month, day, and year of their reports posted.
In the film, the Osaka tower is across the street from Philadelphia's 30th Street station and can be seen a relatively short distance away from the front steps of the hospital. In reality, the hospital used for filming (Allentown State Hospital, 1600 Hanover Ave., Allentown, PA) is not only facing the wrong way, but actually 46 miles from 30th Street station, and so downtown Philadelphia would not be visible from the hospital.
Joseph says the events of Split took place three weeks ago but Casey is not in foster care and her uncle is in prison for assaulting her. He wouldn't even have got a trial date in three weeks.