The last on my list of big-time mistakes is geared to documentary filmmakers. I'll just say, going on a shoot without these is simply a recipe for disaster...
Mistake #5 - Not bringing release forms. Or bringing them and not getting them signed.
The first part is simple and should be drilled into your brain. When you pack for a shoot, always take a bunch of release forms with you.
The second part is pretty simple, too, though inexperienced filmmakers often have a hard time with it. Make sure your subjects sign the releases before you leave.
I always have my subjects sign after the first shoot, just as I'm packing up to leave. (Never before I shoot - I don't want to make them any more self-conscious than they already feel.) I try to make it sound as matter-of-fact as possible. "Oh, sorry, I gotta get your autograph on this or we can't show it anywhere." I've never had anyone refuse me, either. Not even my immediate family.
The only time I've ever had a problem was when I didn't bring them. It can become a huge pain getting their signature later. They've had time to think about the footage that you shot with them, and fret about how awful they must have come across. They're more likely to want to know how you're going to use the footage in the film. Or maybe even say they want to see the footage before signing.
So pack those suckers, and don't forget to get them signed before you leave.