I heard about this website awhile back, and it is pretty neat. It is moviemistakes.com. They have all the mistakes that are in a ton of movies. Most you would never even notice. The mistakes are catagorized too. Some are that you can see the shadow of the boom mike, or in Lord of the Rings, you can see an car traveling on a road in the background.
Since the new Star Wars is out, I checked out the mistakes from it. Be sure to check out the biggest mistakes of all time. They are pretty funny!
Star Wars III:
Best mistakes of all time: