In movies there is sometimes a head shaving seen, often signifying a drastic change in a person. For example when [Lex Luthor gets his head shaven in Superman Vs. Batman](https://m.youtube.com/watch?v=kSG18qgUHhw) or [this scene in 50/50](https://m.youtube.com/watch?v=HqfyKbofF0g). 

I was wondering, if something goes wrong in a scene like this, say an actor sneezes or laughs or the shot is just missed, what do they do then? Does shooting just stop for weeks/months for the hair to grow back? Do they substitute in fake hair? I suppose sometimes the hair is fake to begin with but I imagine lots of times it is real.