Here's a "mistake" that you can retroactively use to lower your taxes in the previous year. Plus, it also boosts your retirement savings! If you have a 401(k) at work, you can also contribute to your retirement savings in a traditional IRA and get a tax deduction if your income falls under the income limits. (For 2012, singles making $58,000 or below and those married filing jointly making $92,000 or below can contribute the full $5,000 to their IRAs; singles with income between $58,000 and $68,000 and married couples with income between $92,000 and $112,000 can make partial contributions.) So let's say that it's 2013, and you realize that you didn't contribute the full amount allowed to you for your IRA for 2012. You can still make a contribution now that will count for 2012, lowering your taxes for that year.