Continuity mistake: As Mitch pulls the window shutters shut and looks at the power cord of the lamp as a possible way to tie it shut there's a candle in a holder to the left of the lamp. In the next shot when he reaches over for the lamp the candle is gone and the next time we see the top of the desk it's not there, or on the floor knocked over.
Continuity mistake: The wire attached to the roof of the Bodega Bay School that the birds sit on as Mitch and Melanie watch was not there earlier in the movie. We see this left corner of the building up close twice earlier in the movie. The first is when Melanie drives up to Annie's and the second even better shot is when Annie stands by her mailbox as Melanie leaves.
Continuity mistake: When Mrs. Brenner escapes the neighbor's farm after finding him dead with eyes pecked out, she is driving so fast that there is a half mile long dust storm trailing behind her pickup truck. She slides almost sideways to a stop in her yard. In the next shot she is seen bolting from the truck... with not a speck of dust stirring anywhere to be seen.