It seems as if Mike was the last to join the seven back in the 1960s. The day Mike joins them was apparently several days after the other kids had encountered It. Out of the discussion about It, which takes place the day of the Rock Battle, we learn that every single kid in the gang has already seen It somewhere. However, later in the movie, Bev tells a story about the blood in her bathroom, and in the flashback, we see all of the 7 kids entering Bev's bathroom to clean the mess up, the day right after the blood had come out of the washbasin.
The Young Beverly draws the slingshot with her right hand, suggesting she's right handed whilst the adult Beverly is left handed.
When Ben briefly flashbacks to Henry Bowers and his gang surrounding him, when we actually see the event, the situation plays out differently, e.g. we hear different sentences.
When the Loser's Club, as adults, all meet in the library, and Mike calls to see where Stan is, when he gets the bad news, Bev gets up from where she was sitting. Moments later, when the camera pulls back in the same scene, she is shown sitting down only to get up again.
At various times - for example, when Audra Denborough first encounters Pennywise - the blue clown makeup triangles above Pennywise's eyes disappear. They're also absent when Pennywise appears in the moving photograph. However, It is a changeling and its clown make-up can be different every time.
Young Bill has a dimple in his chin that old Bill doesn't have, and in all but one scene, the mole on Old Bill's left cheek is not seen on Young Bill.
In the library scene when balloons are popping and spraying blood in peoples' faces, some of the actors visibly flinch, though their characters are not supposed to be aware of the balloons or the blood.
A young Ben mentions Dr. Kildare in 1960 although Dr. Kildare didn't debut until 1961
The flashbacks, involving the characters as children, take place in 1960. The two songs featured to recall that time are It's All Right by The Impressions, which was recorded in 1963, and The Way You Do The Thing You Do by The Temptations, which was recorded in 1964.
In the sewers, the adults watch the paper boat come down the sewer stream, you can see in camera in the shadows as a light illuminates the boat.