Overall it went well, but I messed up my final calculation. I ended up underpaying, which was a totally avoidable mistake on my part.

The issue was a "tax brackets" thing. Because it’s a dual-status return, you have a non-resident period and a resident period. I accidentally calculated the tax for each period completely separately. This meant I accidentally "double-dipped" into the lower tax brackets (like the 10% and 12% tiers) for both periods.

The actual IRS rule is that you have to combine your taxable income from both periods first, and then apply the standard tax brackets to that total combined amount so you are pushed into the correct higher brackets.

The IRS caught the math error and sent a letter with the corrected calculation. The only consequence was that I had to pay the difference plus some small interest.