Thanks to the affordability of digital camcorders, along with the sad fact that it often takes years for documentary filmmakers to scrape together their funding and shoot their stories, most end up accumulating hundreds of hours of footage along the way. Which means you're inevitably in for a very long edit period.
Which all-too-often leads to mistake #2: Hiring your editor before you've raised enough money to pay him or her through the entire edit.
Let's say you've been relatively economical and shot only 150 hours. For argument sake, let's also say that the editor is screening it herself and can get through 5 hours of footage a day (personally, I screen all the footage with my editor and discuss it thoroughly as we go along, so we're lucky to get through 4 hours/day). That still comes to 25 hours/wk, or 6 weeks in total, to slog through the material.
Top NY and LA editors pretty much begin at $3,000/wk, but you have a fantastic project that will save the planet so you've managed to convince one to work at $2,500/wk. So, in this somewhat conservative scenario, you're paying your editor $15,000 just to look at your footage! (Let's not talk about the film I helped shoot and co-produce that dealt with well over 1,000 hours of tape.)
Here's where the mistake comes in. Many producers charge ahead with only partial funding in the blind faith that they'll raise the rest once they have a great rough cut to show. So they get to the cut, or maybe only a partial cut, run out of money, put things on hold and frantically go into fundraising mode.
Do you think the editor is waiting around while they do that? Nope. Will she be available again should the producer actually raise enough money? Possibly. But if she's in demand the far greater likelihood is she soon goes off on another feature.
Your options then are to wait until she's finished, which could be quite a while, or else hire another editor. Who'll likely want to screen most, if not all, of the footage. Which means you've not only lost your original editor and momentum, but you're unnecessarily spending another $15,000.
I'm not unsympathetic to the fact that it could take years to raise enough money to pay a top editor all the way through to the end. That's why so many doc filmmakers wind up editing themselves, at least to begin with. But if you have great ambitions for your film and want to work with the very best editor available, I highly recommend you wait before you leap.