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Are there special exceptions to the rule that (US) capital gains taxes are owed only when the gain materializes?
In addition to the expatriation case already mentioned by Ben Miller, traders/investors are required to use mark-to-market accounting on certain investments.  These go by Section 1256 contracts due to the part of the law  that defines them.  Mark-to-market is also required on straddles (combination of a long and and a short position in equities that are expected to vary inversely to each other). Mark-to-market means that you have to treat the positions as if you closed them at their end-of-year market value (even if you still have the position across the new year).