It doesn't happen very often, but it definitely has happened where I've had a client with a bad wound that is still open, half scabbing, and it needs to be taken care of pronto because you don't want it seen in photos.
So what do you do? You don't want to put makeup on an open wound!
I had never heard of New Skin: Liquid Bandage until Makeup Artist Kim Greene blogged about it. I immediately bought it because I had worked with a male model with a bad wound and sure enough, after I bought it, I worked with a female model who had a very large gash on her leg. So I immediately took out my new skin and gave the scab one coat, let it dry, then a second coat, let it dry, and then put foundation over it. This stuff is amazing! It dries quickly and puts a clear film on top of the wound so that putting makeup on it won't give you an infection of any kind. This eliminated any photoshopping for the photographer.
But this is also great to have in your house! So definitely check it out.