Continuity mistake: When the old man who left town sees the spider return to the cave, it is dark outside. He returns to town to report, and it is daylight outside. The sheriff and his men leave immediately, but when they get to the cave it is dark outside once more. The cave is not so far from town that it takes a day to travel the distance.
Continuity mistake: The size of the spider keeps changing throughout the movie. For instance, at one point the spider is small enough to be dragged into the school auditorium, later it is as big as the Kingmans' house. A minute later it is chasing Kingman, and is only slightly bigger than his car.