If you ask commissioning editors, grants panelists and other lucky blokes who get to sift through countless funding proposals for the most common mistake they come across, this one would probably top them all:
Not having a high enough budget.
Producers, particularly first-time ones, consistently undermine themselves by asking for too little money. And usually far too little.
Why? Because they don't know what typical documentaries actually cost. They underestimate the time involved and tend to vastly underpay themselves. And because a high budget feels so scary to them, they assume it will surely scare off the funders.
In fact, the reverse is true. Nothing sends funders scurrying away faster than producers who under-budget. They see it as a sign that they have no experience and haven't done their homework. They look at a line item like $30,000 (or less!) for a director/producer and wonder how this person will live for a couple of years (yes, they know how long films take) on that little pay. They worry that the film will never get finished. And that, even if it does, it'll be crap.
Drawing up a fair and accurate budget can be a huge psychological barrier to leap across. But there are plenty of books and online resources to reference. And, if you can afford it, I highly recommend attending a pitching forum like the one at Hot Docs as an observer. Along with seeing a few dozen pitches by experienced producers, you get a booklet with a detailed synopsis and budget for each project. You'll see that it's not uncommon for documentary features with international broadcast ambitions to have budgets that range from $500,000 to $800,000. You'll come away with a far better grasp of the documentary marketplace.
And maybe, just maybe, you'll stop undervaluing the contribution you're making to the world with your film.