The Lipnicki gang dries off too quickly after exiting the lake at the quarry.
When Billy falls through the top of the water tower, there is a large hole in it. After the boys rescue him and take him out, the top of the tower is back to the way it was before.
When the family is eating their burnt breakfast, the close-up of the father's plate shows everything except the eggs totally burnt and black. In the next shot, not only his is toast only slightly burnt, but the position of the food on his plate has changed too.
When Moe and Stephen are draining the water from the mine, Moe lifts the valve up to the pipe twice.
Amber's hands after she jumps from the tree with the rope.
When Stephen is telling Stu about his hospitalization, he says that the doctors diagnosed him with post-traumatic stress. That term didn't come about until Post-traumatic Stress Syndrome was included in the DSM-III, nearly ten years after the movie is set.