A friend of mine that is a Canada & US dual citizen got royally screwed by the IRS. Canada has tax free savings accounts (TFSA) that are essentially retirement accounts that are similar to Roth IRA in the US, except with a lot less restrictions. He had opened his TFSA account in Canada when he was 18 and was investing since then. Eventually he got a green card and became a US citizen. It was at that point he realized that the US doesn't consider the TFSA to be a retirement account and that it's fully taxable. This is an account he had prior to moving to the US / having anything to do with the US, and he had to pay taxes on it.

I can't remember if the amount was exempted under $108,700 but I don't think it was given how mad he was that he closed the account because the paper work wasn't work dealing with.

My dad's friend told me a horror story about his friend's mom who was born in the US while he parents were on vacation but she never lived there. The mother is currently retired and is collecting a pension, then she got a letter from the IRS saying that she owed back taxes because she's a US citizen and never filed.