Other mistake: In the scene around the camp fire, Rob Roy can be clearly seen with with plasters on his hand.
Factual error: When the English raid Rob's farm and shoot and kill the cattle, the cattle fall over on their side like toys. Four legged animals shot like that collapse straight down as their legs fail under them.
Factual error: Rob Roy is asked by Montrose to help frame Argyll as a Jacobite. This is the sort of thing the highland aristocracy did all the time, but no-one would have tried to frame the Duke of Argyll. He was the government's main man in the highlands, to whom the framers brought their (real or fake) evidence against their rivals.
Factual error: At one point, Rob Roy refers to the Duke of Argyll as "Your Lordship". Dukes are only ever referred to as "Your Grace" and he is addressed correctly (by Rob Roy and others) throughout the rest of the film.