Did you know that there are people who think that by reducing their income from over $250,000 to under $250,000, they can take home more money? For those of you who aren't aware of this, President Obama is planning to increase taxes on families earning more than $250,000 per year.
Of course, the way the US tax code is set up, the amount of tax you pay as a function of your taxable income is continuous, monotone increasing, and Lipschitz with parameter 1. That is, say that T(x) is the tax due if your taxable income is x. Let y > x. Then T(y) > T(x), and T(y) - T(x) < y - x. As you may note, you can derive from the second of these that
y - T(y) > x - T(x)
which tells us that if you make more money, you get to keep more of your money.
Note that T is actually not differentiable, because it's piecewise linear. Your "tax bracket" is in fact the amount of tax you pay on the last dollar of your income; that is, it's T'(x) where x is your income.
I'm not saying that there are no situations where this sort of thing might make sense. (The tax code is complicated.) But it's certainly not as common as these people would have you believe.
Original article from ABC News; I followed a link from The New York Times via The New Republic.