During a conversation when they cut from one actor to the other it seems like little effort is made in getting the head positioning accurate. One actor will be moving his head to the right or the left or nodding and then when shown from behind while the other guy is talking he’ll be ramrod straight.
It happens so much that I particularly notice if they get it right. Anybody else? Is there any reason why it would be particularly difficult to get right?
Well, the two halves of the conversation could be shot weeks apart, to be pasted together in editing.
Even if they aren’t shot weeks apart, there are probably multiple takes of the scene. If the editor decides that the beginning of take 3 is the best, but he likes the middle of take 7 and the end of take 13, he’ll splice them all together for the final scene. Or maybe the editor decides that none of it was done “right” and calls the actors back to re-do a segment.
If you’ve ever seen a movie shot and edited, it’s a real eye opener. The general public has no idea just how wrong they are about how the process works. Scenes are not shot in any kind of order for the movie - they’re shot based on which days you have each actor, when they particular kinds of makeup, when a set location is available, etc. If you’re filming a TV series, for example, you might shoot every conversation between employee and boss in the office all in one week so that you never need the office set for the rest of the season. Then the actor for the boss, who is only in half of the episodes anyway, only needs to be paid for a week of work and they can go back to scrounging around in dumpsters to make ends meet.
So you can see that any kind of continuity is actually a remarkable challenge.
Forget head positions. It’s the things they interact with that are all-too-often messed up. An umbrella in her left hand, then right, then left again but held differently. In consecutive shots.
Some actors are pretty good about this, some aren’t. Kate Walsh is the worst I’ve ever seen. If she interacts with anything, assume it’s going to jump around.
Most shows are quite good about wardrobe. E.g., if someone is wearing a particular shirt at the start of the day within the show, they wear that shirt at the end of the day. Even if they had to change workclothes in-between. Really hard to manage when scripts are being rewritten on-the-fly and something that was supposed to happen over two days is now in one day. Why wardrobe is so good at this and others aren’t is beyond me.
Don’t know who is to blame when an actor leaves a room with two blue pillows and enters the hallway carrying a blue and brown one. Props? But the actor had to have noticed, right?
You would think.
In the Wizard of Oz, at one point while Dorothy and the Scarecrow are dodging the apple-chucking trees, she’s not wearing the ruby slippers. You’d think that of all the possible props, they’d want to watch continuity on, that would be at the top of the list. Not so, I guess.
Yet I’ve seen this movie a hundred times and never noticed.
The thing I always notice now, and wish I never had, is that when one character is talking and the camera switches to the other character listening, so that you can see the back of the talker’s head in the foreground, the mouth matches the dialog about 10% of the time. Very distracting when you notice.
There’s a scene in Nobody’s Fool, in which Paul Newman and Melanie Griffith are having a conversation. It’s clearly knitted together from different takes, and two camera angles (one pointed at Newman, one at Griffith).
As Newman delivers a line, he puts on his gloves (the movie takes place in the winter, he drives a snowplow, and he’s about to depart). The next shot is Griffith, replying to him. Then, the camera cuts back to Newman, who delivers his next line…as he puts on his gloves. 
Carlos Bernard, who played Tony on 24, didn’t realize until about halfway through the season that the soul patch he was wearing when they started shooting would have to stay on his chin for the duration, as his character would have neither opportunity nor motivation to shave it off midway through the “day.”
it’s a quick shot, blink and you’ll miss it. it happens right after the trees start chucking apples at dorothy and scarecrow, her feet come into the frame for a split second and you can see her wearing the black shoes.
Hell, sometimes they’re not even shot in the same location. The view behind one of the person’s head could be half a world away from the view behind the other person’s head.
Also, if you only see the back of someone’s head in a shot like this, they are often not the real actor but a double standing in while the other performer does their part.
If they intended to only shoot her from the knees up, it might make sense to not wear the special props to reduce the risk of damaging them. Then someone makes a decision at the last minute to pull the camera back and everyone forgets that the actress has the wrong shoes on. That’s just one possible version of the story.
Here’s a fun story from real life that ties into this: An actor brings a leather jacket as part of his character’s costume. It’s his own leather jacket and there’s only one. At some point (in real life, not the script), the jacket sleeve is torn. They had to get a little creative about this, and still wound up with some times when you can tell that the jacket is going from torn to not torn. It’s an interesting clue about the order in which scenes were actually shot.
One of the odder cases of shooting “the same scene” in different places at different times is the park bench scene in Moonlight Mile between Dustin Hoffman and Jake Gyllenhaal. The front and back shots were done in completely different places, meaning that the bench they are sitting on clearly changes.
Filmmakers make weird choices sometimes.
It just surprises me that these movies apparently don’t hire some of these people who are so good at seeing these problems. I guess the return on investment just isn’t high enough.
Still, I bet there’s someone out there online who has or will fix the shoe thing digitally. We’ve been able to do that pretty easily since Jurassic (where they did it when an stunt double accidentally looked at the screen).
They do, they’re called script supervisors. They can’t catch every little thing though.
I do, and it makes me crazy.
It’s also bad on medical shows where the movements of peoples’ mouths when they are wearing surgical masks frequently doesn’t match the dialogue. (Sorry if I’ve also ruined those shows for you.)
They do take voluminous notes, however; the reason things often don’t match is that they’re frequently shot days or weeks apart. They could never get 100% correspondence. 
My favorite discrepancy was in Commando, when Arnie rights the crashed sports car and takes off in it. The side of the auto goes from being smashed in to pristine again in the blink of an eye.
Second favorite is the scene in Diamonds Are Forever where Bond’s Mustang flips from balancing on one side to the other as it emerges from between two buildings.
What I hate is when scenes crucial to the plot end up on the cutting room floor. I spent hours trying to figure out how Plenty O’Toole ended up in Tiffany Case’s swimming pool before I learned that the explanation had been snipped out of the film!
Also earlier in that same scene, the car switches from an early 60’s Porsche with a small spoiler to a mid-70’s model, with a huge whale tail, then back again.