Continuity mistake: In the cemetery scene when Mr. Grimes locks the gates, you can easily see the keys still hanging in the lock as he starts walking back to his truck. In subsequent shots, when Brian is trying to get out, and when his mother arrives, there are no keys hanging in the lock anymore. (01:21:20)
Continuity mistake: Brian rolls on the grass when playing with Fluke. When he starts to roll, he is on the grass about 20 feet from the drive and the grass slopes downhill. In the very next shot, he is only 5 feet away from the grass, and further up the hill.
Continuity mistake: When the wife is in the bathroom after her shower and Fluke is looking at the ring on the table, she puts the ring on, then looks at Fluke. The ring is still on the table in the next shot.