I think this is not necessarily, after all, is acting, what clothing props ah, are fake, help is not careful enough just. In fact, this phenomenon is also common in life, most of the time when filming, a lot of attention is paid to the actor's look of change, and some special effects of the presentation. Many times when we focus on one thing, it is difficult to be distracted to other things, which also causes the situation of treating the details of the observation is not in place.
Even when we watch the show, it is easy to be attracted by the plot of the show, scarcely pay attention to some of the details of the problem, those who wear the scene is not so easy to don't observe. A crew in the finished shooting, there will certainly be special people re-examine these images, but there will always be visual fatigue, observation is not careful is also a common thing.
Leaving aside this, for ourselves, in our daily lives, we often make such mistakes, for example, in the company to write copy, they repeatedly pondered, repeatedly review is not found problems, especially in the article of the misspelling, and even let a few together to see also did not find out. Waiting for the copy of the public, soon to be sharp-eyed colleagues found that there are a variety of small problems in the copy, and then let yourself look at the time, is indeed their own mistakes, which can be what can be done ah, but also very helpless.
I think the existence of movie and TV dramas and this is a class of things, and the movie and TV drama is a huge project, even if again meticulous also have some observation is not meticulous place. But the professional crew can still try to avoid these low-level errors, after all, such a shot is to make a joke, it must take a lot of effort.