5. Overlooking unearned income
Because your Social Security number exists on bank and investment accounts, the IRS knows precisely how much unearned income you made as soon as you did, thanks to the 1099 forms that financial institutions send to the tax agency. If you forget to include this info on your return, the IRS examiners will let you know that you owe taxes on it, too. And depending on when your oversight is discovered, you also could owe penalties and interest on the unreported earnings.
Also, be careful in figuring any tax due on your investment earnings. Some dividends, reported to you and the IRS on Form 1099-DIV, are eligible for lower capital gains tax rates. These amounts are in box 1b (qualified dividends) of that form. You'll have to do some additional work to compute this tax amount, especially if you're filing your returns without the help of a tax software program, but it's worth the effort to shave some dollars off your eventual tax bill.