Meeting a residency test in two countries is one problem. Having both countries actually assess tax on you as a resident, in the same year, is a different and more urgent one. When that happens, you're no longer just planning around the rules — you're in an active dispute, and the clock on how you respond matters as much as the facts themselves.

If you split your time between the U.S. and another country — or moved abroad but kept ties back home — you may meet the legal test for tax residency in both places at once. Dual residency is not automatically a problem, but it is a warning sign that your U.S. filing could be wrong if it is not addressed, and it can lead to the same income being reported as taxable in two countries with no relief unless you claim it correctly.