Continuity mistake: When Bullock is warning Reeves of the coming winter blizzard, she is looking through pictures that depicted her and her friends enjoying the snow. She then turns to the dog and asks "Remember that day? You were not happy". However it's revealed that both characters have the same dog, given to Bullock's boyfriend by Reeves, so when the snow happened in Reeves' time - 2004 - the dog was with him.
Continuity mistake: During the opening credits, Kate leaves the Lake House via the kitchen, where we see the indoor tree is branches only-no leaves. Then from her car she looks back to the house, where we now see that tree is covered in orange leaves.
Other mistake: Every time somebody climbs to the attic in the "lake house", you see a small, dark, enclosed chamber with some abandoned stuff. However, as soon as you see the "lake house" from the outside, it is a single-level construction entirely made of glass, with no inside staircase or ladder, and three smaller glasshouses on the roof. There is not a single wooden panel to enclose the attic, nor any construction in the house that looks like it could be that attic.
Suggested correction: It's not a "dark, enclosed chamber"; it's a glass surround with part of the windows darkened, but others letting light in. The ladder is shown near the end of the movie (when he's putting the box of letters away) to fold up into a small space, and the "attic" is merely a small space, easily the height of the roof plus one of the glass bits you can see from outside the house.
After looking at many photos of the house, there is no space for the attic.
Audio problem: Alexis confirms in a conversation with his brother that it's Valentine's day 2006, but if you watch his lips he says 2004.
Factual error: When Kate is at Il Mare waiting for Alex, she straightens the knives at his place. The knives are set incorrectly, with the blades facing away from the plate. The fanciest restaurant in Chicago would never make this mistake. (01:11:45)
Plot hole: When Kate went to the aid of the man lying in the street, she was very upset about his death. Would she not have got his name being that she works in a hospital? Also she could have easily Googled him in 2006 to find out where he was or that he had died already. It seemed an obvious ending that she would save his life to change history.
Continuity mistake: During the scene when Alex and Mona are riding in the truck with Jack the dog in the back end, the dog vanishes during several shots in which she should be visible.
Continuity mistake: In the lunch scene where Sandra is showing her mom the letter she says the year is 2006. The poster behind her is advertising tickets for the year 2004.
Revealing mistake: Kate's birthday is July 10th. When she is (with long hair) outside on the patio with Alex, you can see their breath as they talk, and there is not a living thing in the back yard, it's all dormant (as in late fall/winter).
Continuity mistake: When Sandra Bullock is at the table in her mother's kitchen, she has her hand to her chin during a close-up, but when the camera angle changes, both hands are on the table.
Other mistake: In the hospital, when Alex's father takes the cup of coffee, it is obvious that there is nothing in the cup. He tilts it quite far back - if it had been full of coffee, most of it would have poured down his chest.
Revealing mistake: When Kate kisses Morgan in front of 1620 N. Racine, the camera moves up through the branches of a tree. You can see one of the lower branches shake when it is bumped by the crane. (01:17:20)
Audio problem: Near the end when Alex is talking to his brother on Valentine's Day, the overdubbed audio says the date is 2006 and not 2004, like Keanu originally said. His lips say 2004, which shouldn't have been overdubbed since Alex passed two years prior. (01:28:35)
