"For instance, in Fellowship there's a car in the background of one shot in the field. For the extended, they took it out,"

That is ashame. Mistakes like that in movies are always fun. You know it is just a movie anyway, I really don't think the car took away from anybodies enjoyment of the movie, if they even managed to see it in the first place. Well, at least they didn't go the Lucas way and magnify the the car twenty times so you couldn't help but notice it (like the stormtrooper hitting his head had an exagerated sound effect placed for the special editions. Making what was once something cool to look for, something annoyingly silly.)