As a filmmaker, you always want to go for the best shot possible. So you plan ahead and try to get everything in position right down to the last detail. You start shooting and everything is going as planned and you leave that day thinking you did an amazing job. Then you go over the footage and you find out that someone was standing in the frame that shouldn't have been there. AHHHHHH!
This is actually something that is very common, especially among new filmmakers and amateurs. Everyone who starts out makes some sort of mistake in this area at one point or another. I've even made those mistake in the beginning. The funny thing is, you're so focused on one area of the shot that you forget to look at everything else that is going on. Sometimes, the story that's being told in the background is just as important as what's going on in the main dialog.
There was a project recently that I was working on and I was so focused on what I needed to focus on that I almost forgot to look around what's behind the subject. I looked around at the 2nd take and I noticed that there was someone in the frame that shouldn't be there. Now, there's nothing wrong with bringing your friends along to see what's going on and the actors bringing their crew and family along too, but there does need to be clear communication as to where everyone can stand. Had I had noticed that sooner, I would have had 2 complete shots to work with. I could still work with it no problem, it's just going to be a little more challenging in the editing process.
In the same project, I had a guy that was just starting out in this line of work and he wanted to get some experience and I decided that he could help me in my project so he can get that experience. I was working with my shot and I noticed that he was a little too close to the subject and he eventually ended up in my shot. Normally I would have been all over him and said you need to stay in your area and focus on what I told you, but instead I decided to wait till later to show him what he did wrong. When it came time to edit the project, he came over to see how I do things and when it came time for the shot that he was in, I didn't say anything, I just let him see it. He was so surprised by how it was turning out, and all of a sudden, there he was. He saw himself in a very vital point in the project and I remember him saying, "Oh crap, I ruined the shot! I'm sorry." I told him it was ok because I knew how to fix it. Some simple rotoscoping and photoshop did the trick, but as I was fixing the shot, I was telling him about how important it was to be aware of what's going on around you and not just what your filming.
All in all, the project ended up good. I was satisfied, the customer was satisfied and no one knew about the mistake that almost ruined the entire thing. The main thing to remember is, Always remember to look around you and your subject. It'll save you some heartache in the editing process.