[jupp0r]

It's totally illegal in Germany to lie on tax forms. Whether you'll be sentenced for an honest mistake or not is a different matter, I don't think you would in either jurisdiction if it's something minor.

[junon]

Lying and making a mistake are two different things. The IRS penalizes you for both.

[jupp0r]

The IRS sends you a notice to correct your return if they find a mistake. You fill out the form and send it back. Matter solved.

If it's a simple arithmetic error, the IRS will sometimes silentliy fix it for you and you'll never know.