If you’re like most taxpayers, you probably have a couple primary fears this time of year: overpaying (or underpaying) the government and being audited. But are those worries warranted?

It depends on how carefully you complete your returns.

According to the IRS, only about 1 percent of U.S. taxpayers on average face an income tax audit each year, but that number is growing for some groups (namely those earning more than $200,000 annually). And experts say there are certain filing mistakes that put you at a greater risk of getting audited, as well as paying an unnecessarily high tax bill.

Here are five of the most common and costly blunders, and how to avoid them this year.

1. Not reporting all of your income

If you’re a full-time employee, your company should withhold taxes on your behalf and report your income on your W-2. If that’s your only income stream, then you’re all set. But if you make any other money throughout the year, like through a side job or investment dividends, you may receive one or more Form 1099s indicating your earnings.

The problem: Some people fail to report this “extra” money because they either never receive a 1099 or receive it after filing their return.

“You should include on your tax return all the taxable income you earned, whether you received a 1099 or not,” warns Steven Warren, a Certified Public Accountant in Minneapolis. “If you did not include some taxable income and you became aware of the income by receiving a 1099 after you filed, you normally should file amended federal and state returns.”

You can do that by completing Form 1040X. If you don’t—and you owe significantly more than was stated on your original return—you’ll be charged interest and penalties on the unpaid amount.

When shouldn’t you amend? Because the IRS only states that they’ll impose penalties for “substantial understatement of tax” without offering a specific threshold, that’s up for interpretation.

But know this before you decide not to amend: Whenever you’re issued a 1099—even if it comes late and it’s for a minuscule amount—the IRS receives a copy, too. While the agency may not come after you for failing to report a few dollars of interest earned on your savings account, say, it has the option to do so.

2. Not reporting stock sales

“We are constantly helping clients respond to tax notices for forgotten sales of stock transactions,” says Gail Rosen, a Certified Public Accountant in Martinsville, N.J. “It doesn’t matter if you later reinvested that money. All sales of stock, excluding those in a retirement account, must be reported.”

The IRS receives copies of stock sales throughout the year, so just like 1099s, it won’t slip their mind—even if it does yours. What’s more, the IRS’s default assumption is that, unless you show otherwise, stock sales should be treated as a short-term capital gain. That assumes you held the stock for less than a year and the sale should be taxed at your income tax level.

If you had the stock for more than a year,  you’ll want to claim it as a long-term capital gain, so you’re taxed less. You can report that on a Form 1040, schedule D. You’ll just need to provide the IRS with the original amount you paid for the stock and the price at which you sold it. And be absolutely sure that you sold the stock at least a year after you bought it.

If you forget to report the sales, the paperwork is not the only headache, Rosen says. This oversight can also cost you back taxes, plus interest and penalties.

3. Not taking the deductions and credits you’re owed

If you qualify for deductions and credits but don’t claim them, you’re writing Uncle Sam a bigger check than you have to—a mistake Rosen sees fairly often. That’s partly because there are a lot more deductions than many people realize. Did you know, for example, that in addition to deducting charitable deductions, you can also deduct 14 cents for every mile that you drove while volunteering for charity?

Home-office deductions trip people up, too. “Many people avoid this deduction because they’re afraid it might trigger an audit,” Rosen says. “But they should not ignore it if they are legally entitled because it can save a lot of tax dollars.”

Case in point: Rosen had a new client who works from home as an architect, yet has never claimed this deduction. As a result, Rosen estimates he sacrificed more than $6,300 in savings for 2014.

4. Taking deductions and credits for which you don’t qualify

On the other hand, taking deductions you don’t legally qualify for will make you vulnerable to an audit and penalties.

For example, Rosen often sees people incorrectly deducting costs for businesses that haven’t been launched. “Small businesses are often not aware that any expenses incurred before the first sale are startup costs, and these cannot be deducted,” she says. “[After the first sale], they are deducted over 15 years, and you can elect to deduct the first $5,000 in the first year of business.”

If you try to deduct disallowed business expenses, all of your expenses could be off the table. “They do not even have to audit each expense,” Rosen says. “You will owe taxes, plus interest and possibly penalties on the back of these expenses.”

If you realize you’ve incorrectly taken a deduction or credit, correct the error on an amended return. “Knowingly taking a deduction you are not entitled to may be [considered] fraud,” Warren says. “And fraud leaves you open to large penalties and a never-ending statute of limitations.”

5. Waiting too late to file

Filing after the deadline—that’s April 17, 2018—is a very costly mistake. Even if you can’t afford to pay your entire tax bill, it’s important to file and pay something.

If your return is more than 60 days late, you’ll now pay a minimum of $205 or 100 percent of the taxes owed, whichever is less. The penalty for failing to file (or filing late) is about 5 percent per month on any taxes owed, up to 25 percent. The penalty for failing to pay is generally .5 percent per month on your unpaid taxes, also up to 25 percent. When both penalties apply, the IRS will cap the penalties at 5 percent per month.

“The penalty rate is applied to the amount owed, so that’s why it’s important to pay as much as you can,” Warren says. “Likewise, it is important to file even when you can’t pay [the entire amount] to avoid the larger of the two penalties.”

(You may not have to pay a failure-to-file or failure-to-pay penalty if you can show “reasonable cause” for not filing or paying on time.)

If you need more time, file for an extension—but remember that just gives you extra time to file; your payment is still due in April.

To avoid any issues, it’s often wise to file as soon as you’ve received all your tax documents. When you push it off until the last minute, it’s easy to rush through the process and  make mistakes. “When you miss items or do things incorrectly, this can cause an audit,” Rosen says. “Start working on your taxes early. Then put it down, look at it again and get a professional if you need the help.”

This article was updated in December 2017.