Factual error: The mansion Allie is staying in over the summer is Boone Hall Plantation in South Carolina, near Charleston. It was built in the 1930's, so factually the house couldn't have been in the family as long as the characters make it seem. A lot of the filming was also done in Kiawah, an island near Folly Beach.
Continuity mistake: When Allie has returned to Noah and they are on the boat in the area with all of the swans, her hair is in a bun. Before it starts raining in the next scene, her hair is only half up and curled as if it had been down all along.
Factual error: When Allie is looking at the moon before entering the house with Noah, there are a few contrails barely visible in the sky, but there couldn't have been any contrails back in those days because planes didn't fly that high or give off the same exhaust as modern-day planes.
Continuity mistake: When Allie crashes her car into the fence, Noah has a beer in his hand. As Allie swings the car around, you can see Noah in the background and the beer has disappeared.
Revealing mistake: When Noah and Allie meet outside the movie theatre and later dance in the intersection of the streets, the viewer can see their breath. It is very cold outside and Noah is wearing a heavy jacket. Summer in South Carolina is oppressively hot, even at night.
Audio problem: In one of the beginning scenes, when Noah and Fin are at the fair, you see Allie and Sarah on the bumper cars. Allie bumps into Sarah, and you can see and hear Sarah yell "Rachel," which is actually the name of the actress who plays Allie - Rachel McAdams.
Visible crew/equipment: When Allie gets back into her car, after her first visit to Noah's house, you can see a microphone - crew member on the back of the exterior mirror.
Continuity mistake: When Allie and Noah are making love after the seven years when they fall at the feather pillow, notice at the back there is a white book shelf which is at the right and only can be seen half of it. A few seconds later in the same camera position the book shelf is at the centre.
Continuity mistake: When Allie drives off after she gets her letters and she and Noah argue, Noah walks significantly past the back of the car and puts his hands on his head. When Allie gets in, he's standing up by the front bumper still with his hands on his head.
Audio problem: When Noah and Allie are at the dilapidated old house, when Allie says 'This place is gigantic' her words don't match her mouth moving.
Continuity mistake: When Noah hears that Ally has gone he goes to her house and there is a cement driveway but in all other scenes at the house the driveway is gravel.
Continuity mistake: When Noah and Allie are preparing to make love for the first time, they slowly undress and lower down onto the floor. The front shot of Allie shows that her hair is tied back in a Pollyanna hairstyle, but when she kneels down, the back shot shows that her hair is completely out.