For such a neat freak, Wasabi's safety precautions around his laser slicing device in the lab are shockingly weak. Anyone walking through it (which Hiro was about to do) would suffer an instant and incredibly gruesome death, and one could easily lose a finger or a limb by being too careless, yet the only safety precaution is yellow tape on the floor.
In the warehouse, Hiro holds a broom as a weapon as he approaches the barrel of microbots, but when he comes closer the broom is nowhere to be found, and there is no sound to indicate that he dropped it.
When Callahan first meets Hiro, he is seen wearing a blue shirt, but when he asks Hiro for is robot, we can see his sleeve is red in color. When we see a full shot of Callahan later, his shirt, and sleeves, are blue again.
When Hiro and Baymax are at the police office, you can see Baymax putting three pieces of tape on on each arm and you only see him take 6 pieces of tape. Yet, when the frame switches, he has a lot more pieces of tape on his hands, arms, and belly.
When we see Hiro's room for the first time, we can see 2 figures on the top of his computer. However, in the shot right after Tadashi's death, the top of his computer is bare. Then, after Hiro and Baymax return from the police station, the 2 figures are on the computer again.
When Honey meets Hiro, she has earbuds in her ears. She starts talking to Hiro, pulls them out of her ears, and they dangle out of her shirt. The dangling earbuds then disappear in the close-up shot of her and Hiro.
When the vehicle speedily crashes into the ocean, Hiro's seat belt is jammed. At the bed of the ocean, the viewers are shown Hiro attempting to unlock his seat belt. When Baymax surfaces with the rest of the characters, Hiro is no longer restrained by his seat belt without explanation.
When Tadashi rescues Hiro from the bot fight, Hiro puts Megabot between him and Tadashi on the scooter. You can see Megabot's face is red briefly. When Tadashi jumps the scooter and Hiro looks at his reflection, Megabot's face is yellow. A moment later when Tadashi and Hiro are talking, you can see that Megabot's face is red again.
When Hiro visits Tadashi's school Go Go enters on her bike and hangs it up, removes a magnetic wheel for a moment, and then tosses it back. As it locks into position it slowly rotates clockwise, but when the scene cuts to a closeup of Hiro looking at it the wheel is rotating in the opposite direction.
When Baymax scans Hiro the second time, he states that he has no injuries and that the only diagnosis was "puberty". However, if you look at the blood pressure, it is 113/90 which is considered hypertension (high diastolic blood pressure). Anyone with health care knowledge would recognize as a problem.
Mistaken as a goof: When Hiro first meets everyone at the college, Honey says her ball of carbide weighs 400 pounds. In reality, a ball of carbide measuring 36" in diameter (judging size from the film) would weigh over 14,000 pounds. This is not necessarily a goof, as it is not stated that the ball of carbide is solid.
After encountering Yokai, Baymax states that he scans the health-care needs of anyone he meets and therefore has noted the bio-metrics of the villain. Being created in Professor Callahan's lab, Baymax would have certainly encountered the Professor previously and should have been able to see immediately that the two had identical bio-metrics.
What type of item gets sucked in to the portal or not is very inconsistent. Some items are sucked in with great force while similar items in the same shot are not affected. The team hides behind a piece of debris that doesn't seem different from the debris being sucked in to the portal, and not only is it motionless but it seems to shield them from the effects of the portal. They can behave normally without any noticeable force acting on them. Earlier, Hiro was not shielded from the effects of the portal even though he was indoors (he was drawn up against the ceiling by the upward force) and much farther away from the portal.
The goof items below may give away important plot points.
During his farewell to Baymax, a tear is seen sliding down Hiro's cheek. Tears cannot form in zero-gravity, which the free-floating debris implies exists inside the portal, since there is no gravitational pull there to act upon them.
The entirety of the final confrontation could have been avoided had Hiro used Baymax's scanner to track Callaghan before Krei's grand opening ceremony. The team discovers the truth about his daughter late at night, giving them plenty of time to have tracked him down and apprehended him.