Continuity mistake: After escaping from hanging, Robin heads for the castle gate. The first time we see it, the portcullis rope Robin later cuts is hanging away from the castle in a nice loop. In the next shot it is hanging vertically, but reverts to being a loop again for the rest of the scene.
Continuity mistake: Robin, in his escape from the scaffold, pauses on his horse before the portcullis and exhorts his men to return to Sherwood. He then rides over to the right, draws a sword from behind him and slashes through the rope which activates the portcullis and rides the rope to the top of the wall. No sword is seen on the horse when he is exhorting his men.
Continuity mistake: In the final swordfight, Basil Rathbone lunges past Errol Flynn, who dodges him and leaps to the floor below. Rathbone continues on dropping his sword at the base of the steps. In the next cut, however, the sword lies between Rathbone and Flynn - a good eight feet away - allowing Robin Hood to pick it up gallantly and offer it back.
Revealing mistake: At the end, when Guy loses the sword fight with Robin Hood, his body topples over a parapet and falls to the stone floor below. In the initial shot of him falling, it's a stunt man (who apparently broke his ankle in the process, according to the dvd commentary), but the next shot of the body landing makes the use of a dummy a little too obvious: the "corpse" bounces back up several inches before settling to the floor.
Continuity mistake: In the feast in the forest scene which occurs after Robin Hood and his men capture Sir Guy's entourage, Robin sits next to Marian and offers her a roasted bird. She refuses the bird, but when the camera cuts back to Robin he has a mutton leg. Next the camera goes back to Marian who looks at Robin who now has the bird again.