Continuity mistake: When Grace and Ms. Mills are sitting in the living room you can see a clock on the right, but in the next shot it's disappeared.
Continuity mistake: Before both kids go out by the window, Anne is painting, and the paint shows different patterns in each shot. (01:21:25)
Continuity mistake: When the father comes home, the scars on his face are very small - in the bed scene they are a lot bigger. (00:55:45 - 01:04:20)
Continuity mistake: Anna's eyes keep changing throughout the whole movie. One scene they're brown, another they're blue, then they go to brown again.
Continuity mistake: When Grace is explaining the children's photosensitivity to Mrs. Mills, she lights an oil lamp, yet a large shadow of the picture frame then appears directly above the flame. This would seem to be cast by a handheld lamp somewhere off to camera right.
Continuity mistake: When Anne and Nicolas are at the dining room table having soup, you can see that Anne's spoon is next to the bowl and not in the soup itself while she is reading and not eating. A few seconds later she still has the book in her hands, but now the spoon is in the soup. (00:24:05)