I watched part of a scene for a very small movie being filmed at my cousin's house a few years ago.
To tape one scene, literally took all day. What they were filming was a person walking 15 meters or so from the bitumen on the road, into the front yard, and up the stairs to then say his lines, he met another person at the top of the stairs.
There was a stand-in for most of the day, who did the walk (the actors turned up later in the day). When the cameramen had the camera angles perfect, the smoothness of the camera dolly running perfectly, the walk completed perfectly and smoothly by the stand-in, the walk up the stairs completed perfectly - this took (so it seemed, we went shopping in the middle of it all, as it was a bit drawn out, lol) six or more hours, then the actor arrived and acted. The actor then completed the walk (he had to continue to re-do the walk and re-deliver his lines, too, which he did really well). The actor couldn't be spoken to before acting in the scene, he stood there doing breathing exercises for ages, waiting for the camera to reset up, and for the filmed scene to be watched and approved - or not. Actually, the actor acted so well, when I watched him acting in the scene, I nearly cried, although it was of course make believe (the actor was that convincing in his sadness - I peaked out from the curtain inside the house, out of camera shot, lol).
It was interesting to watch, I wouldn't want to be watching daily - boy, it took so much time to do very little.
I can see why the inconsistencies in filming due to the above.
These days, (in this instance of filming a scene, anyway) there is a person employed to make sure that there is continuity in placement of whatever needs to be consistent, with all scenes.