I suspect failing to report significant income to the tax authorities would be considered tax fraud in just about any legislation. If there weren't any kind of

I suspect failing to report significant income to the tax authorities would be considered tax fraud in just about any legislation. If there weren't any kind of a potential penalty for failing to report or for significantly under-reporting, doing so would be potentially beneficial with no drawbacks. Failing to report income or reporting false information for financial gain can lead to extra tax or prosecution for tax fraud where I live. I'd definitely be careful to report all income if I had income from sources that don't automatically withhold taxes, especially if it were significant. I don't think they'll drag you to criminal court if you make a small mistake, though. But if you fail to report thousands of euros of income and the authorities get wind of it, sure, especially if it seems intentional. I don't know if the risk of prosecution or other legal consequences is somehow greater in the US.