You can also get caught up if you move between countries that have tax treaties but where the tax years don't align on the calendar and where the schedule for

You can also get caught up if you move between countries that have tax treaties but where the tax years don't align on the calendar and where the schedule for paying estimated taxes are different, like the US and the UK. The US tax year runs from Jan - Dec, with a quarterly estimated tax payment schedule for self-employed people. In the UK, it runs from Apr - Mar, with bi-annual tax bills. So there's a point at the end of the calendar year where you owe US taxes (that you should be able to fully wipe out with tax paid to the UK), but the UK hasn't calculated the tax amount yet nor asked for the money yet, so you can't show the US that you already paid those taxes under the tax treaty. So you end up owing two countries taxes on the same money, at least temporarily. tl;dr - everything sucks, hire an accountant