Goofs

(at around 1h 29 mins) The plaster cast on Jay's arm gets replaced with athletic wrapping tape for the pool scene towards the end of the movie. When she crawls out of the pool, she bends her "broken" wrist nearly 90 degrees, which the cast would have kept completely immobilized. The hard cast returns for later scenes.

When Jay leaves the classroom at 29:06, her backpack is clearly unzipped, but when she gets out of the class, it is zipped at 29:24. There is no way she could've zipped her backpack between the two scenes.

(at around 58 mins) When the characters are sitting at the beach, Jay's clothing changes position between shots of Paul drinking a soda and when she is shown again.

Though the movie, the shots that covered driving around town were obviously shot in late fall and winter (bare trees, occasionally leaves on the ground), but the exteriors with dramatic action all took place in the summer (green trees, obviously warm nights).

(at around 1h 29 mins) Near the end of the film, Paul fires a small handgun several times and hits "It" in the head, knocking him out. The scene clearly reveals that bullets hit the pool surface at an approximate angle of 30 degrees, and judging by the actors' height, the bullets travel at least nine to 10 feet in the water before they hit the target. It would be impossible for those bullets, fired at that extreme angle, to wound someone after traveling so far in the water. In fact, MythBusters (2003) did an episode investigating the effects of guns fired at targets that are underwater, proving that water provides protection.

(at around 1h 23 mins) In setting up the items to be pushed into the pool, they plug some of them into a surge suppressor, which would actually prevent those items from causing a shock. ============ Correction: A surge suppressor does not in any way help prevent shocks (and/or electrocution). The device that does is called (in the U.S.) a Ground Fault Interruptor (GFI) and is electrically and functionally VERY different from a surge suppressor. A surge suppressor protects attached electrical equipment from limited types and levels of power variations (transients and surges), not shocks. A GFI protects people.

(at around 1h 20 mins) "It" would have no reason to climb on Jay's roof, always taking the most direct route to the cursed, through a door or window. However, when talking to Hugh/Jeff he says that "It" is still taunting him despite Jay being the most recent target. If Jay had sex with one or more of the men on the boat, as implied but never directly stated, then this would be a similar taunt.

(at around 1h 12 mins) When Greg opens the door to what appears to be his mom, her left breast wasn't visible in the shot where she knocks on the door, but when he opens the door, it is visible.

However, It is illusory, continually changing appearance. Minutes before this, It appeared as Greg himself. Earlier it changes from a young woman, to a very tall man, to a little boy within moments.

(at around 20 mins) When Hugh brings Jay to the abandoned parking structure, The Entity, in the form of a naked woman, is seen at the base of the structure wearing sandals. When the entity appears on the upper level of the parking structure moments later, it is barefoot. It had plenty of time to cast off the sandals.

(at around 1h 2 mins) The station wagon's column mounted shifter is in the park position when Jay is escaping the lake house scene and crashing into the corn field.

(at around 57 mins) When Jay is practicing firing the revolver, the revolving cylinder holding the rounds never revolves to put a fresh round in front of the firing pin.

(at around 8 mins) In the bedroom, the camera is reflected in the TV set.

If all Jeff knows is that some one-night stand gave "It" to him, how does he know so much about "It"? And more importantly, how does he know "It" could hurt him if "It" touched him? How does he know how to pass "It" on and that "It" all has to be explained to the person he passes "It" to and that "It" would come after him if the person died? None of that makes any sense.

(at around 10 mins) When Hugh and Jay are at the movies playing Hugh's game, Hugh points out that he would want to be a child in the room; he says it would be nice to wear a diaper and Jay says "He's probably shitting his pants right now." The shot of the child shows he's five to eight years old, clearly too old to wear diapers.