My tax refund almost went down under. My wife and I moved down here to Australia last year, and recently had to file our domestic income taxes for the first time. I let my wife do it, since she always did our taxes back in the States. On a hunch, I decided to double check her numbers before she filed them here, and I am glad I did. Out of habit, she used an IRS calculator she had bookmarked, when in fact she needed to use an ATO tax calculator.
She laughed and cried at the same time when I pointed out what she did. We both called local friends for the best ato tax calculator to use and found it. Once we reran our tax numbers using an actual Australian Taxation Office calculator, we went from owing money to being due a small refund. That of course made us very happy.