JAMES TARANTO: 8960 or Fight! Can’t file your taxes? Blame ObamaCare.

Some TurboTax customers are mad at Intuit, maker of the popular tax-prep software, because they’ve finished their returns but are unable to file. Their anger is misplaced. They should blame the Internal Revenue Service, along with the 111th Congress and President Obama for enacting and signing the tax increase with which TurboTax can’t yet comply. (They could also blame George W. Bush if they’re in a jocose frame of mind.)

At issue is ObamaCare’s new 3.8% “net investment income tax.” It took effect Jan. 1, 2013, so that taxpayers are encountering it just now as they prepare their returns for last year. In effect, it applies the Medicare payroll tax to interest, dividends and capital gains.

But it doesn’t apply to all such income. If your modified adjusted gross income is under $200,000 (or $250,000 for a married couple), you don’t pay the tax at all. Further, if your modified AGI is above the threshold but your noninvestment income is below it, the tax is applied on the difference between your total income and the threshold.

If that’s hard to follow, here’s an example: Suppose you’re an unmarried taxpayer with a modified AGI of $210,000 and investment income of $20,000. Your net investment income is $10,000, the portion of your investment income above the total income threshold of $200,000. Your net investment income tax is 3.8% of this sum, or $380.

If you owe net investment income, you have to complete a single-page Form 8960 to calculate your modified AGI and the tax. But the form’s brevity belies the new tax’s complexity.

Read the whole thing.