It takes a small army of people to organize and execute a successful event. Sitting at the top is the event planner, but under them is a roster filled with venue staff, participants and third-party vendors. Even the most seasoned planner can find it overwhelming to manage all of these different groups, and when it comes to tracking expenses it helps to have a sharp eye for detail.
One of the difficult parts to managing an event budget is predicting where all of the costs will come from. The vision for what the event will be and how it actually turns out almost always has a few differences. Each subtle change has an impact on the budget, which is where it really pays to project as many costs as possible.
Of course every planner tries to account for all of their event expenses, but when you're beginning with a blank sheet of paper it is not easy to think of everything. Recently I published a list of event planning expenses that can provide a good starting point if you need help.