Factual error: During the opening scene in 1988, a silver TV from the late 90's/early 00's is in the living room/dining room area. (00:02:10)
Suggested correction: There were several TVs similar to the one pictured in 1988. A quick Google search of "TV's available in 1988" will tell you this. Without knowing the manufacturer or model of the TV, you cannot state this as a movie mistake. If it was a LCD TV in 1988 then yes but, this one, no you can't assume this TV was not available.
Factual error: Nivea Soft Cream is on the shelves at the chemist - this did not exist in 1989, when the film is set.
Suggested correction: I think the date is subject to debate. The only thing we really have to go on is it's 2019 in chapter 2, and It comes back every 27 years which would be 1992.
The date is not subject to debate. The marquee on the movie theater is advertising both "Batman" and "Lethal Weapon 2," placing the movie in the summer of 1989.
There is no debate about the date. After the title card it says "June 1989." The opening scene took place "October 1988."
Continuity mistake: When Bev is hanging in the air, she is wearing a dress, with no leggings. As she gets pulled down with the help of the boys, she is wearing dark coloured leggings.
Suggested correction: Bev wears that dress in several scenes and the only time she isn't wearing leggings is at the quarry.