Gaffes

When Tom is cake tasting, the very first shot shows the Tiffany-box cake already part eaten, then in the next few shots it disappears until the lady brings a fresh Tiffany-box cake to the table. Tasting one cake after another.

Violet's outfit changes during the viewing of the donut testing participants. She starts in the same outfit, changes to another, then is suddenly back in the original outfit.

During Tom's conversation with his parents at breakfast, Tom picks up his fork and in the next shot he picks it up again. His mom's drink also changes positions and levels throughout the meal.

The morning after Tom and Violet's engagement party, Tom's hand is inconsistently holding to Violet's arm in succeeding sequences.

When Tom & Violet are together in London she has bangs. Presumably a few days later when they're in San Francisco, Violet does not have bangs, and her hair length doesn't show any sign that she recently had them.

A crossbow propels its bolt with tremendous power than can shatter bone. If a hunting bolt, it may have a barbed tip which is designed to cause more damage if it is just pulled out and not surgically removed. She seemed to recover in weeks when it should be the better part of a year.

The man that compliments Tom's tacos in his food truck says he's had tacos in Buenos Aires, although tacos aren't traditional or very common at all in Argentina. Except for Steak Tacos, Shrimp Tacos and Fish Tacos.

In the opening engagement scene, in his speech Jason Segel's character said that they can return it to Tiffany's if she doesn't like the ring. The engagement ring box is red, which looks identical to a Cartier box, which is a different jewelry store.

Throughout the film, Violet and her colleagues refer to people taking part in their psychology experiments as "subjects". This term is no longer used in psychology (and has not been used for decades) as it is thought to be disrespectful and has unethical, dehumanising connotations. Rather, today psychologists use the term "participant" to refer to people who take part in an experiment.

When Violet attends the film's final funeral - said to be in the UK - it is very clear from the architecture of the buildings and the road features that the location is not the UK, and is definitely North America.

At 1:58:26 they are clearly traveling west through Broadway Tunnel then turn right on Larkin. When they pull over, you can clearly see SF City Hall in the background. Those two locales are approximately 2 miles apart. Not to mention, when they turn right on Larkin, SF City Hall is in the opposite direction.