In Canada, people can put money into a tax-deferred retirement fund (RRSP); the income tax is paid only when the money is withdrawn (sometimes 30 or 40 years later). The US does not recognize the RRSP as deserving of a tax-deferred status, but as a regular savings account. (https://www.tnvisaexpert.com/articles/how-canadian-rrsps-tax...) Thus some dual US-Canada citizens suddenly have faced significant tax bills from the US. This includes people that have never set foot in the US since they were toddlers.