During a scene set in 2004, the Paul McCartney song "This Never Happened to Me Before" is featured prominently. This song is on the album "Chaos and Creation in the Backyard," which was released on September 13, 2005.
The taxicabs in the film have the same color license plates in 2004 and 2006, even though Illinois taxi license plates are reissued annually with different colors. The blue plates shown in the film are the 2005 series.
When Alex catches up with his dog, Jack, at Morgan and Kate's house, Alex tells Morgan that Jack is a she, not a he. A few moments later, Alex's girlfriend, Mona, catches up with Alex and his dog. She says about the dog, "Oh, I see you found him."
When Alex shows his brother the lake house in 2004, he mentions that the house has no way to reach the water and that there should be a staircase so that you could go swimming. He adds the stairs himself later on. However, when Alex receives the book of his father's memoirs, he looks at a photo of his father and himself, a child, at the lake house. There are clearly stairs leading down to the water underneath the house, but the stairs did not exist until Alex added them in 2004.
When Alex first meets Morgan at his house in 2004, there are dead leaves all over the yard of Morgan's house like it's the fall, but all the trees around are bright green and it's obviously spring- some of the trees are just starting to sprout leaves.
When Alex Wyler (Keanu Reeves) finds out that his dad is in the hospital he gets to the front desk to ask about his Dad's room. In that scene his sweat shirt was zipped up but when the attendant Anna Klyczynski (Shohreh Aghdashloo) realises he is the son of Simon Wyler ('Christopher Plummer') and comes round to talk to him his zipper is down.
When Alex meets Morgan he's holding two cases of beer, one is clearly Samuel Adams and another case which is a black box with small white writing on it. After Alex ask if he needs helps, neither is carrying the Samuel Adams box, and Alex is carrying a Dogwood box which is black but has different pictures and writing than the other case Morgan was carrying.
When Mona is showing Alex her boots in the truck, the first shot is from the truck bed into the truck. There is a visible rear view mirror. Subsequent shots are from the front of the truck and the inside of the truck, and there is no rear view mirror.
When Alex is at the hospital talking to the doctor about his dad, his sweatshirt is zipped when the scene was shot from the side, and unzipped in the scene that is shot from the front, and then zipped again from the side again.
Kate puts her red coat on the back of the chair twice in the architects office near the end. Once when 3 people are in view, and a second time when there are 4 people in the room. A female assistant joins them.
When Kate arrives at the bar she asks for a glass of wine. Immediately after her fellow resident doctor arrives and though we have not seen Kate drinking that much, in the next shot the glass is nearly empty.
In the scene where Kate is at the kitchen table, talking to her mother about her mother's first love, there is a cutaway to Kate's face, right hand under her chin. When it cuts back, she is holding a knife and fork with two hands.
The address that Kate (Sandra Bullock) lives at in 2006 - 1620 North Racine Street - is a non-existent address. While there is a North Racine Street in Chicago, it does not run in the 1600N block of the city.
Kate and Alex are supposed to be living exactly two years apart throughout the film which is not possible because 2004 was a leap year, 2006 wasn't. Once it got to February 28th for both, the next day would be February 29th for Alex but March 1st for Kate.
Kate takes the Amtrak train out of the Riverside station to Madison, however the only trains that stop at the Riverside station are Metra trains to and from Chicago only, no Amtrak trains stop there. Further, Amtrak does not have service to Madison. As of 2011, there is no train service between Chicago and Madison.
Kate stands on an L platform with a sign that reads "Purple Line to Linden board here", but the train that pulls up has a Loop destination sign--the opposite end of the line. Loop-bound trains should stop on the other side of the platform.
In Kate's first letter to Alex she mentions the paw prints and a box in the attic. The lake house is a flat roof design, except for the sliding greenhouse roof above the tree. There is no space for a slanted attic space.
Alex and Kate sharing a day together is a great idea, but it could not have been Saturday for both of them. The same date 2 years apart never fall on the same day of the week; they are at least 2 (if a leap year's involved, 3) days apart on the calendar.
Before Kate sends the red scarf to Alex, she discusses the snow storm coming and says to the dog 'remember that day you were not happy' but the dog was supposedly with Alex in his time already and couldn't possibly have been with Kate during that snow storm.
Kate and Morgan are in the apartment. Kate is annoyed and stomps over to the bedroom area and breaks loose a piece of the hardwood. Hidden there, in a white paper bag, is the book that Alex retrieved for her when Kate had left it at the train station in 2004. Yet we never see the scene on how Alex could have possibly been able to put the book away under the hardwood floor.
In 2006 Kate writes Alex about the dog and tells Alex that she has named the dog Jack. In 2004 when Alex reads this letter, he calls out "Jack!" and the dog responds by jumping up and barking, indicating that she knows her name. However, Kate hasn't met the dog yet, so the dog shouldn't have known the name Jack.
When Kate is with her mother at the diner, she is sitting in front of a poster for an event taking place in December 2004 but she is in 2006. This could be an old poster just kept on the wall but it seems like it may be an oversight.
The goof item below may give away important plot points.
On Valentine's Day, 2006, Kate tries desperately to save the man's life, but he dies. She spends several hours looking at him, so she should know his face well. Later, she is told by Alex that he was the man she kissed at her birthday party in 2004, so she should remember him as the man that she tried to save. Despite all of this, she's still hurt when he doesn't show up for their date in 2006, and still doesn't remember him as the man that she tried to save, but he died after all.
Kate asks Alex to pick up her book that she forgot at the Train Station 2 years previous, and to send it back to her. He ends up placing it under the floorboards of her bedroom that his construction company is building. There's no reason why he couldn't have put it in the Lake House mailbox, like she does when she send him his Father's book after he dies.