From Newsweek, December 1 2003 - I couldn't find the original version online, but found this at http://www.theonering.com/reading-room/critical-viewpoints/blunders.
Gaffes: Fellowship of the Nitpickers
A Web site traces Peter Jackson's every step-and misstep
Dec. 1 issue - It ain't easy making movies. Tiny blunders-the kind normal folks don't notice-get made all the time. But nothing escapes the hawks at moviemistakes.com. For kicks, NEWSWEEK ran the site's best catches past Peter Jackson to get his side of the screw-up.
'THE FELLOWSHIP OF THE RING'
Blunder No. 1: "During the scene with Sam and Frodo in the field with a scarecrow, you can plainly see a car cruising past in the distance, from left to right."
Jackson: We actually didn't know about the car until we were cutting the movie. The smoke [from the exhaust] and dust wasn't so bad because there was already lots of it around, but the bloody windshield was reflecting the sun back into the camera lens. So we erased it for the DVD. I think some people were upset because they tried to show it to their friends and it was gone.
Blunder No. 2: "While Arwen is carrying Frodo to the Ford, a close-up of his face shows his eyes and mouth covered in a green, pus-type substance. Moments later, his face is clean."
Jackson: Yeah, we started with the pus and then we got just a bit revolted by it. So we eased back on the pus. We didn't think Elijah looked very good with pus.
Blunder No. 3: "When Arwen and Frodo are being chased on horseback by the Ringwraiths, the soundtrack to the scene is a cantering horse. A canter is three beats, whereas a gallop-which is what the horses on screen are doing-is four very fast beats that often sound like a single beat."
Jackson: I should've-well, it's too late to fire anyone. The damage has been done.
Blunder No. 4: "When the hobbits enter Bree, there's a distance shot from above in which the principal actors have clearly been replaced by shorter doubles. Also, the sizes of the doubles are completely wrong. The last hobbit into Bree is really, really fat-and he isn't even the double for Sam, who's the stockiest of the hobbits. It's actually Merry, who's very thin."
Jackson: [ Giggles ] It's true. There are definitely little doubles in that shot, and we did have four standard hobbits who were all about four feet high. So if you're really paying attention, there are shots where you can sense that someone's body shape is suddenly slightly different.
Blunder No. 5: "During the scene in which the hobbits ask Strider where he's taking them, he answers, 'Into the wild.' A second later, as Viggo Mortensen walks past the camera, the bow he carries on his back bumps into the camera, nudging the screen a bit."
Jackson: It does, yeah. But it was the best take. We did three or four takes, and for various reasons his movement past the camera just wasn't as dynamic. So I chose the one that has a little bit of a bump. I was just hoping people wouldn't notice. [ Laughs ] This is fun.
'THE TWO TOWERS'
Blunder No. 1: "As people are fleeing Edoras, there are many shots of Eowyn-and in all of them, her hairstyle is different."
Jackson: [ Laughs ] That shouldn't be! Maybe it was the wind. It was really windy there.
Blunder No. 2: "The Uruk-hai are these huge, ferocious, twisted, unbelievably strong warriors-and yet the hobbits keep laying them out by tossing rocks at them."
Jackson: In the books, hobbits are renowned for stone-throwing, so I guess if you're going to rely on somebody to bring down an Uruk-hai with a stone, it'd be a hobbit.
Blunder No. 3: "When Saruman is talking to Sauron through the palantir, his lips aren't moving."
Jackson: Well, that's because he's engaged in a psychic session. That was deliberate.
Blunder No. 4: "In shots of Fangorn Forest from a distance, it's an evergreen forest. Seen from up close or inside, it's a deciduous forest."
Jackson: Wow. Well, when you see it from the outside, it's a real beech forest on the South Island of New Zealand. But seen from the inside, it's a miniature forest that we built. [ Pause ] You've got pages and pages there. And those are all mistakes they've spotted?