Imagine watching Superman with a physicist who can't help himself but to tell you that "That's physically impossible" as Superman leaps over a tall building in a single bound.
Imagine watching Pirates of the Carribean with a geographer who keeps insisting that the Isle de Muerte is not located where Captain Jack Sparrow says it is.
Imagine watching Finding Nemo with a zoologist who won't shut up about the fact that fish can't speak English.
Now, imagine watching Braveheart with a historian who can resist the temptation to tell you that actually, Wallace and his followers were Lowlanders and wouldn't have worn kilts, that Wallace couldn't possibly have sired the future King Edward III - who was born 7 years after Wallace's death, or that the Irish's role in the film is totally fictional - to name but three of a thousand complaints historians have with the film.
 Elizabeth Ewan. "Braveheart" American Historial Review 100, no. 4 (October 1995):1220 (And there are a hundred more where that came from).