When you set out to make a film, and you are facing the budget, that little thing that controls everything about the film, the first and probably only things that you think of are the film crew's salaries, all the equipment, and location costs. Sure these are important to keep track of in your budgeting, but the problem is that if you are aware of these costs, you can control them. But, it's the unseen costs that can derail your film, even the seemingly trivial costs.
Some examples of unseen, trivial costs are damage to rental equipment. Or late fees for those rentals. To prepare for these, stick close to your time schedule, and put people in charge of your rented equipment who will take very good care of it.
An often overlooked cost involves location shooting. Filmmakers can't just shoot anywhere and everywhere they choose. If they think this way, there will be fines that will upset your budget. Location permits and fees should make up at least 10% of your film budget.
Everything is relative, especially with film budgets. This means that, if you have a large budget, you should also plan for an equally large unplanned expenses line item. It doesn't mean that you need to spend all the money allocated here. It means that if you are careful, you can save a lot of money.
Independent filmmakers, those who especially work with no money budgets, are used to treating their equipment with kid gloves and dealing with unexpected costs. If you are not an independent, and you actually have a budget, you should still think as if you working with a zero dollar budget. Keeping watch constantly over your expenses will help keep costs down through awareness of spending. Know your limitations and stay within them. Spend money set up for the unexpected on only the unexpected costs, because if you spend it elsewhere, it won't be around when you do need it.
