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Are there special exceptions to the rule that (US) capital gains taxes are owed only when the gain materializes?
Normally, you don't pay capital gains tax until you actually realize a capital gain.  However, there are some exceptions. The exception that affected Eduardo Saverin is the expatriation tax, or exit tax.  If you leave a country and are no longer a tax resident, your former country taxes you on your unrealized capital gains from the period that you were a tax resident of that country.  There are several countries that have an expatriation tax, including the United States.  Saverin left the U.S. before the Facebook IPO. Saverin was perhaps already planning on leaving the U.S. (he is originally from Brazil and has investments in Asia), so leaving before the IPO limited the amount of capital gains tax he had to pay upon his exit.  (Source: Wall Street Journal: So How Much Did He Really Save?) Another situation that might be considered an exception and affects a lot of us is capital gain distributions inside a mutual fund.  When mutual fund managers sell investments inside the fund and realize gains, they have to distribute those gains among all the mutual fund investors.  This often takes the form of additional shares of the mutual fund that you are given, and you have to pay capital gains tax on these distributions.  As a result, you can invest in a mutual fund, leave your money there and not sell, but have to pay capital gains tax anyway.  In fact, you could owe capital gains tax on the distributions even if the value of your mutual fund investment has gone down.