When Harold refuses to accept the gift of the box of cookies, the box disappears from the counter top between shots.
Before Harold and Professor Hilbert are talking about pancakes, the same man (with a dark denim jacket) walks by in the same direction twice. Then during the pancakes, a woman with a zip up brown sweatshirt walks towards them and pass them, twice.
When Harold and Dave eat dinner and discuss what one would do if they knew they were going to die, as the camera cuts back and forth there appears to be a piece of melon in the middle of the table or on one of the plates and it comes and goes depending on the shot.
When Harold and Ana are talking in the bakery and she has made him cookies, one camera is on Ana and there are approximately 5 cookies on the tray; the camera flashes to Harold, then back to Ana, and there are only 3 cookies on the tray.
As Harold rides the bus reading the "Death and Taxes" manuscript, an Asian woman is seated behind him. Her arm/sleeve is visible in shots involving only Harold. Alternating shots of Harold and the back of the bus inconsistently show the woman - she vanishes and reappears. Her sleeve remains consistently visible.
Harold Crick read's Karen Eiffel's "Death and Taxes" novel all in one read on the bus. He's the last one on the bus as it goes into the garage to get cleaned. In real life, people aren't allowed on the bus when it goes into the garage to get cleaned and/or repaired. The bus driver will tell everyone to get off the bus.
Ana Pascal says to Howard about Harvard Law, "I was barely accepted, I mean, really, barely. The only reason they let me come was because of my essay". College admissions do not explain to students why they are accepted or rejected.
Federal law prohibits IRS employees from accessing their own records or the records of friends or relatives. So, when Harold started dating Ana, he should have removed himself from her tax case.
Near the beginning, Ms. Eifel dictates "When asked by a co-worker for the product of 67 and 453, Harold drew a blank. He quickly answered 30,351 despite the answer really being 31,305." The product of 67 and 453 actually is 30,351. This was meant to get viewers to question who was dictating Harold's life, the narrator or Harold himself. It wouldn't be a coincidence that the "incorrect" number given by Harold would in fact be the correct answer.
When the crane smashes through the wall of Harrold's apartment, he is terrified and jumps on top of the sofa. The camera switches to another angle, and for a few seconds you can see a woman through a window strolling casually down the hallway of the next building only a few feet away. If a crane had just smashed through a wall, she would have immediately stopped to look, or ran. She doesn't even acknowledge what is going on.
When Dr. Hilbert meets with Harold in his office after reading the book, a crew member is reflected in the window.
Harold's survival evidently weakens the novel, but the film fails to consider the long process of editing/rewrites between writing and publication. Assuming the first draft is canon for Harold's life (as viewers are led to believe) Eiffel should've been able to safely restore the original ending after the fact.
Harold tells Ana that she can deduct the bread she gives away to the homeless as charitable contributions on her tax return, which is not true. In order to qualify as a charitable contribution, the property (in this case, the bread) must be given to a qualified charitable organization and be properly documented. Simply giving away food to the homeless is not enough, as there would be no way to prove that she actually gave the bread away.
Harold meets Ana on the bus and gets off 27 blocks earlier than his stop. After realizing this, he starts walking in the direction the bus came from, instead of the way it was going.
In his counseling of Harold, Hilbert defines comedy and tragedy as prose fiction when they are, in fact, dramatic genres.
When Ms. Eiffel walks out of the fruit store, there's a sign in Spanish that reads "DENERO" instead of "DINERO" (money).
The Wristwatch saves Harold Crick's life by stopping the bleeding of the artery in his right arm by a shard of metal. Actually the Wristwatch should have killed him because a clogged artery could lead to some serious blood clotting and cause a major stroke or heart attack.