Double-Check Your Tax Homework

Tick, tock -- April 15 is around the corner. It's getting down to the wire, which is exactly when taxpayers make simple yet devastating mistakes filling out their tax returns.

Sadly, there's no spell-check-like function for tax forms (though if you do them with tax software, at least the computer hamsters will likely catch egregious math errors).

The best defense against tax return errors is taking your time, reviewing prior-year returns, and familiarizing yourself with the tax rules. No, it's not an ideal way to spend a weekend, but it's certainly better than being on the wrong side of the IRS.

I asked longtime CPA (and Motley Fool tax guru) Roy Lewis for a cheat sheet on the most common tax return flubs. Before you lick the envelope and mail your return to Uncle Sam, make sure you haven't made any of these common boo-boos:

Missing Social Security numbers: You know your number by heart, but you also need to know the Social Security numbers of any dependents you include on your return or risk having the deduction denied. If you married or changed your last name this year, make sure the proper authorities (namely the Social Security administration) know. The IRS and Social Security databases are integrated, so if a number is attached to an "incorrect" name, your return could be rejected.

Sloppy math: Don't rush through the computations. Many of the numbers you input affect other figures on your return. One small addition or subtraction error can reverberate and really mess things up.

Ignoring the Alternative Minimum Tax (AMT): AMT is complex enough to make even a tax pro's head spin. If you don't know what it is or if it applies to you, do some digging. The IRS will flag an AMT that’s MIA, and you could get smacked with the taxes you owe plus penalties and interest if you don't pay your tab on time.

Leaving money on the table: Some common tax return errors actually work in your favor, not Uncle Sam's. Many people assume that itemizing your deductions is the best way to reduce your tax bill, but don't automatically dismiss the standard deduction. If your itemized deductions aren't even close (e.g., if your home is paid off or you live in a state that doesn't charge income tax), then going with the standard deduction may be better for your bottom line.

Committing state tax refund oopsies: Don't make the mistake many taxpayers do by blindly reporting prior-year state tax deductions as income in the current year. Roy says that even if the state taxing authorities notified you of your refund, they have no idea whether that refund is taxable. If you didn't receive a benefit for deducting those taxes last year, your refund may be partially or completely untaxable. "For example, you may have used the standard deduction for federal purposes and itemized on your state return, or used the sales tax tables rather than state taxes paid in the prior year. If that's the case, you might be overstating your taxable income by simply reporting your entire state tax refund as current-year income," he says.

Overlooking carry-forwards: Sometimes lemons (like a stock you own that tanked) can be turned into lemonade (a tax deduction!). And in many cases, you can carry forward losses from prior years to this year's tax return -- so don't forget about them. (As if they were in danger of slipping your mind this year.)

Fool.com writer Dayana Yochim's favorite homeopathic sleep aid is the IRS's Tax Code.