Tom and Rachel are shown playing on a Nintendo Wii game console on day 11. Near the end of the film, the narrator says day 500 is Wednesday, May 23. The last time May 23 was a Wednesday was in 2007. Counting back, this means day 11 would have been January 18, 2006. The Nintendo Wii was not released until November 19, 2006.
On day 303 early in the film, Tom receives an e-mail from Summer that she can't meet that week and hopes that means he's ready to be friends. The e-mail is dated May 7, 2008. On Day 500, Tom awaits an interview and the narrator says it was May 23. This is inconsistent with the rest of the movie which seems to be set in 2006 and early 2007.
At the beginning, Day 8, during the office party Tom asks Summer about moving "here" and how long she has been "here." She answers "since Saturday" which doesn't make sense as it is Day 8 since they met which is more than a week.
The day in which the characters go to The Mill (the karaoke bar) is a Friday, as stated by McKenzie. This also happens to be day 28. If Tom and Summer met on 8th January 2006, which was a Sunday, then day 28 would've taken place on 4th February, which was a Saturday, not a Friday as McKenzie had stated.
For someone who is supposedly a trained architect, Tom's drawings display an odd discrepancy in perspective. Perspective issues are a beginner mistake an architect would have been trained to avoid during the first semester in architecture school.
In the split shots from the Expectations/Reality scene Summer wears an engagement ring in the Reality point-of-view, which is seen first when she hugs Tom at the door. When he enters the apartment the shot is the same for both Expectation and Reality and in both Summer is no longer wearing the ring.
During Millie's engagement party Tom lifts his wine glass to drink, but just before the glass reaches his mouth the scene cuts to another angle and his hand is suddenly by his waist again and he lifts the glass to drink again.
When Tom and his sister Rachel are sitting on the little stand.They're discussing Tom and Summer's relationship. Just before she leaves she has her left wrist over her right wrist but in the next shot she has her hands clasped.
On Day 402 while riding the train on the way to a wedding, Tom crosses paths with Summer in the same train car. All of the seats and passengers are facing the camera. However, the motion outside the windows shows the scenery moving the other way so that the car would be traveling backwards.
Summer's heart shaped birthmark on the left side of her neck is is shown twice (same footage) but is then not seen in situations where it should clearly be visible (most notably during the wedding scene where she is dancing with Tom).