Tax season began on January 23, but millions of Americans have likely put off filing until the last few weeks before the April 18 deadline.

Government cutbacks last year resulted in a number of layoffs at the IRS. Likely as a result, the number of income tax audits fell in 2016 for the sixth straight year. Still, about 1 million people were audited last year. This does not include filers who had their returns sent back to be corrected — often for extremely simple and preventable reasons.

On its website, the IRS lists several common errors filers make each year. The errors vary in complexity from failing to correctly calculate a deduction on a 1090A form to simply failing to sign and date the return or even using the wrong postage. While some of these mistakes seem obvious and easily avoidable, they are common enough that the IRS lists them as issues.

Click here to see the 19 most common tax mistakes.

A great many of these errors can be avoided simply by filing electronically, as most Americans now do. The number and share of Americans filing electronically rose from 68.5 million, or about 51%, in 2005 to 128.8 million, or 91%, in 2015. Still, filing through a software such as turbotax, or using a service that e-files for you, can be expensive, and some prefer the sense of control that filing by hand can provide. Electronic filing also can also prevent filers from including written explanations for certain deductions.

Even when e-filing, many of these mistakes still remain common. Many of these can be lumped into the category of, simply, failing to fill in the right boxes or the correct figures, or forgetting to include the various deductions, credits, and allowances.

These are the 19 most common tax mistakes.