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.
In the intro talking about Harold's life they say he counts the stroke of brushing his teeth each direction to 38 times. Then when the movie actually shows him brushing his teeth he only does it 24 times per up and down. In addition, when trying to provide his own narration, he appears to give a total of 72 (not 76).
Early in the movie Ms. Eiffel dictates that Harold always ties his tie in the movie in a single-bow Windsor knot. That is not always the case. The bow is in a double bow at least once after that. When it is in a single bow, the bow is not always tied on the same side - which is usually the case.
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.
A little before the time that Harold and Professor Hilbert are talking about pancakes, you can see the same man (with a dark denim jacket) walk by in the same direction twice. Then during the pancake scene, you can see a woman with a zip up brown sweatshirt walk towards them and pass them, twice.
When Harold and Ana are talking in the bakery and she has made him cookies, one scene 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.
When Harold and Professor Hilbert start going over Hilbert's checklist of tragic heroes, Hilbert is eating a cup of yogurt. A moment after Hilbert takes his first spoon of yogurt, the camera cuts away. When it comes back again, there's a spot of yogurt on his tie. When the camera cuts away and later returns, it is gone, and moments later, it is back on the tie again.
Just before Professor Hilbert and Harold start going over Hilbert's checklist of tragic heroes, Hilbert asks Harold to be seated. Harold clearly sits on a jacket that is thrown on the chair. In later moments of the scene the jacket is neatly placed on the back of the couch next to Harold.
When Harold and Prof. Hilbert are in Hilbert's office talking about being the 'king' of anything, there is a shot when Harold is seen with his arms on his legs. In the next shot, he is seen with his arms resting on the arms of the chair. In following shot, he is seen with his arms on his legs again.
In the scene where 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 Ana Pascal and Harold Crick are kissing on the couch, after she says "I want you too," he slips her jacket down to her elbows. When the shot changes, her jacket sleeve is only on one arm, but when the camera changes angles, her jacket is on both arms again.
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.
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. Was it 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.
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.