Continuity mistake: In the scene at the college where Callaghan asks to look at Hiro's Megabot, Callaghan's shirt is blue, but when he holds out his arm to take Megabot, his shirt sleeve is red. Later on he is wearing a red shirt at the SFTT. (00:13:30)
Other mistake: In the garage when completing his invention there are 12 blue bins full. But only 5 characters have a bin each entering the building, but there are 6 bins when they are chatting before the inverters show. (00:16:29)
Factual error: When the car goes to the bottom of the ocean after the chase, the water in it is raising as if leaking in, despite the fact that the door on Hiro's side is missing and the water should be rushing in. (00:50:15)
Continuity mistake: At the end, Baymax rocket-glove changes model. When Baymax puts his Rocket-glove at the thruster on the ship it has the original model seen earlier in the movie, however, when the ship exits the space through the portal and the glow shuts-down it has another model. (01:27:00 - 01:29:00)
Continuity mistake: When Honey meets Hiro, she has earbuds in her ears playing music. She starts talking to Hiro, pulls them out of her ears, and they dangle out of her shirt. The dangling earbuds disappear in the close up shot of her and Hiro.
Other mistake: Honey touches the pink ball and it explodes into a pink puff of smoke. Despite standing in the smoke with Honey, Hiro does not get any of the pink on him, whereas Honey is covered in it.
Character mistake: Baymax, a healthcare robot, says that his programming includes a database of 10,000 symptoms of disease. Yet, he issues the warning, "After eating, wait one hour before swimming." That warning, often given by parents to their children, has no basis in medical fact. People who do not get cramps after eating will not develop cramps by going swimming after eating: http://www.snopes.com/oldwives/hourwait.asp.
Continuity mistake: When Hiro and Baymax are in the warehouse the single microbot is let go by Hiro before they are attacked, linking with the others. A few minutes later, the microbot is seen back in the Petri dish again before they are attacked.