A long bull market brings blessings—and a curse. An investor who shelled out around $1,500 for Apple stock back in early 2001 is sitting on a position worth nearly $2.2 million today, including dividends. Someone who inherited $100,000 of Nvidia stock in early 2019 has around $5.3 million today. The curse? You'd like to spread your bets by selling at least a portion of such lopsided winners. But that would trigger a titanic tax bill, unless the holding is in a retirement account.

Even if, like Wall Street Journal reader Marc Gordon, you bought an index fund or two and "let it ride" for a couple of decades, you can be under the same curse. "Since the vast majority of my portfolio's value is due to unrealized capital gains, taxes will be horrible if I sell," he emailed me recently. Gordon, who lives in California, says state and federal taxes could eat up roughly one-third of his long-term gains if he sells.

Maybe you can't ever imagine holding millions of dollars in a few stocks, but an inheritance windfall could easily put you in a similar position.