The New York Times has an amusing "Budget Puzzle" that allows you to select various options for budget cuts and tax increases at the federal level, in an attempt to close the anticipated US federal deficits for 2015 and 2030. The visible math is basic addition and subtraction, but hidden below the projected results of each option are various unstated assumptions. Many of these assumptions will be economic in nature, and economists have been known to make some entertaining assumptions. (Do you remember that bit in Econ 101 about the ideal production level for a firm being the quantity where marginal cost equals marginal revenue? Do you recall any mention of a capacity limit?)
Also missing from the NYT puzzle are projections of indirect costs for any of the options. If we reduce our military presence in Afghanistan drastically, will we subsequently need to make an expensive redeployment there? If we increase taxes on employer-paid health care policies, will employers reduce the availability of those policies and, if so, will that indirectly impose greater health care costs on the federal government?
It's obvious why the Times puzzle does not go into those considerations -- they're involve considerable uncertainty, and they would probably make the puzzle too complicated to be entertaining. As OR people, we're used to making assumptions (frequently designed to simplify a real-world mess into a tractable problem); but we should also be acutely aware that the puzzle contains many unstated assumptions, and not attach too much credibility to the results.
Incidentally, I balanced the budget with 34% spending cuts and 66% tax increases, and I consider myself to be a fiscal conservative (but a realist, at least in the relative topology of mathematicians).