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.
Character mistake: When Mr. Rogue is spinning the globe to assign countries to the students he spins for Albert Morio, and he puts his finger just west of North America and says "Lebanon." Lebanon is in Asia, slightly northeast of Egypt and we can see Egypt on the globe on the other side of where his finger is at. He's nowhere near Lebanon.
Character mistake: When giving Eggsy a tour, Galahad tells him that Oxfords are any formal shoe with open lacing. This is incorrect as Oxfords have closed lacing.
Character mistake: When the police come to find Tammy, they say they are looking for a 2004 Cadillac Deville. It is a 1997 Seville in the movie.
Character mistake: At the end of the movie, when Jaggu is listening to the sounds recorded by PK, the audio contains the question which she asked, "Tumhare planet pe sab aise hi hote, bade kan wale?" but does not contain the answer which PK gave, even though he was sitting right there. This seems to suggest that PK did not exist in the real world.