When Will Scarlet finds the injured Much in Sherwood Forest about three quarters of the way through the movie, a white vehicle can be seen travelling from right to left in the background.
Listen carefully to the sounds the swords make during the final battle at Nottingham castle. When Robin and Sir Guy are fighting, they sound different (clanging as opposed to clicking) than all the other combatants. The reason for the discrepancy is simple. Errol Flynn and Basil Rathbone, both experienced performers, are using real steel swords for their one-on-one fight scenes, while everyone else is using plastic props.