During the archery contest, Robin shots his final arrow and the sheriff knocks it up into the sky. Robin shoots again and corrects the arrows path. It hits the bullseye and split the sheriff's arrow in two, which cannot be done from this angle.
Hello! IT'S A CARTOON!!!!! Can you be any more picky? If you really want to talk about reality, let's not for get that the actors are animals, that talk! And they walk on two feet. Not many foxes, rabbits, turtles, etc. I know can do that. We also ought to mention the scene where Robin (a fox) wanders into a rabbits den and they welcome him. And how about Marian (also a fox) having a chicken as her attendant. This is not supposed to be real. IT IS A CARTOON!!! I don't think I can stress that enough.
I don't think it's worth marking down, really... it's not physically possible, no, but the point is that Robin is THAT MUCH BETTER than anyone else. Cartoons have never been constrained by the laws of mere physics! ;)
If you watch carefully the arrow that Robin Hood sends down, doesn't hit the sheriffs arrow. It hits the bulls eye then seconds after the sheriffs arrow kinda withers. As you said it would be impossible to split in the middle =)
Ok, how exciting would a cartoon be if everything was realistic and true to life? Would that scene have been as neat if Robin Hood just shot the arrow like a normal person? NO!! That's the whole point of a cartoon, to do things that you can't do in real life. I honestly think that the Disney animation/production team is smart enough to know that the arrow situation you described defies the laws of physics, etc. Therefore I seriously doubt it's a slip up.