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.
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.
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.
Answer: In her apartment in Chicago. Because Alex knew her address, he hid it in the floorboard for her to eventually find. He knew how important the book was to her. He didn't want to send it to her in the mail, as she originally requested.