The first time Stan recites the entire Boy Scout Oath, he misses the line "To obey the Scout Law". This line goes between the lines "To do my duty to God and my country" and "To help other people at all times".
When Mike is in front of the class, he says that the disaster in 1900 had to do with the Derry Sandpipe. When he is older and explaining to the group a disaster every 30 years, he says that the 1900 disaster was the massacre at Drake's Creek, not at the Derry Sandpipe.
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.
When Beverly is using the slingshot, the order of bottles she shoots change. For example, there is one white bottle left that she shoots, then there are none left. However, the last bottle she shoots is another white bottle.
At the beginning of the film we see a tray of China cups filled up with water, and it is clear that rain is pouring onto them. Then a second later, the little girl on her bike appears, and you can see that it isn't raining at all her clothes are completely dry, as are her mothers.
When Stan is reading the "Atlanta Register", he is wearing slip-on black dress shoes. When his wife finds him in the bathroom and sees his clothes on the toilet, the shoes beside the toilet (that he was wearing) are clearly black lace-ups.
When "He thrusts his fists against the posts and still insists he sees the ghosts." is being typed on the typewriter in the library, it is aligned going down the middle the short segments, but when Ben holds the paper and starts reading it, you can see through the back of it that it it written in lines across the top of the page.
After Bev has been chased out of her fathers house 30 years later, If you look to the far side of the screen you can faintly see the thin metal stick which they tied to the balloon so that one of the crew could make it bounce from a distance.
In the scene where the last six are in the library and the books are flying off the shelf, look in-between the top and second shelves and you will faintly see a flash of a metal object running the length of the books. It appears to be the device to knock the books flying off the shelves.
When Richie sees his epitaph on a cinema billboard, it says he was born in 1950. The events of his childhood take place in 1960, making him ten years old, not twelve years old like he and the rest of the gang are supposed to be.
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 it's clown make-up can be different every time.