When The Manticore is telling Laurel about the curse, she tears off her sleeves to reveal expository tattoos on her arms. However, when Laurel initially asks about the curse, The Manticore's left sleeve is already seen to be torn off as the tattoo is already visible.
Early in the movie, Ian sits on a bench across the street from the high school. That bench is never seen again.
When Officer Bronco discovers the bumper to the van on the path he drives on the path without moving or running over the bumper.
At 34:15 in the movie, when Ian and Barley are deciding what route to take to get to the gem, Dad taps his foot on Barley's foot. The foot appears to change abruptly from his right foot to his left foot.
When Officer Bronco catches up with the boys at the bridge, he removes his sunglasses and has them in his hand. Officer Bronco then uses that hand to gesture at the boys and the sunglasses are not in his hand.
In Onward, when Ian sticks his hand out the car window the second time, his hand appears to have human skin.