As a general rule of thumb, an unexpected letter from the Social Security Administration won’t make you happy. That’s if you can even understand what it says.

It’s hard to know just how many Minnesotans who lean on Social Security payments for daily living costs got a particularly disappointing letter around Thanksgiving. It was the one that tried to explain how monthly benefit payments were going to be a lot lower next year because they made too much money in 2015.

Before jumping to the conclusion that anybody who made enough money a couple of years ago to see their Social Security benefits get cut deserves little sympathy, keep reading.

It is possible that a person could report a lot of taxable income in 2015 and still have a no better than middle-class retiree lifestyle. It is possible to think an $8,000 cut in benefits next year is going to be very, very painful.

There are more than a few older Minnesotans who in the last few weeks have found out that’s their situation — because they are longtime holders of Medtronic stock.

Medtronic is today Medtronic PLC, still run out of Fridley but formally based around the corner from the National Concert Hall in Dublin, Ireland. The process by which Medtronic Inc. common stock was turned into ordinary shares of Medtronic PLC created a tax liability for shareholders in 2015 that is still costing some of them money.

The need to exchange old Medtronic stock for new Medtronic stock created a big tax bill for some retirees, many of whom accumulated stock through the company’s stock purchase plan while they were employees. If you were hired in 1986, your first shares would have cost about 68 cents each, adjusted for all the stock splits since then. In 10 years you would have made about 20 times your money on those shares. Over 20 years, you would have made close to 85 times your money.

That was the rocket ride at Medtronic during this period as it turned itself into a global consolidator and dominant player. Sales and earnings growth later became harder to come by, but what to do with Medtronic stock that has appreciated nearly a hundredfold is a nice problem to have. One thing’s certain, though: by selling the shares the holder would have generated a whopping capital gain.

That explains why older people had been sitting on them. They weren’t savvy 80-somethings who somehow kept their appetite for the kind of investment risk that comes with a concentrated stock position.

“I had someone in her 90s, been holding onto the stock forever with the plan for the children to inherit someday,” said Lori Hume, who heads private client services for the accounting and consulting firm Wipfli LLP. “All of a sudden she ends up with this huge gain. There was just nothing you could do about it.”

The merger deal blew up a financial plan that made a lot of sense, too. Passing the stock down meant heirs would get the one-time opportunity for what’s called a step-up in basis, free to simply sell with a gain or loss pegged to the stock price the day Mom or Dad died. Now you can imagine how it became possible for retirees to reach their golden years relying on the usual sorts of middle-class sources of income like Social Security and distributions from retirement plans and still have a half-million dollars or more of Medtronic stock.

We know of these situations because we’ve heard from the shareholders, both at the time Medtronic acquired Covidien and ended up based in Ireland and just in the last couple of weeks. One was an 84-year-old Minnesotan now living in Arizona who only worked at Medtronic 10 years, but those were 10 particularly good years to be there.

She was savvy enough to realize right after the deal was announced in 2014 that it would be taxable, eventually paying more than $115,000 in cash for a noncash capital gain. She admits to not foreseeing the letter that she and her husband just received that said their Social Security benefits would be cut nearly $8,000 next year.

Hume heard this story and said that the situation is more complicated than Social Security benefits getting cut. Social Security was trying to explain an increase in the cost of one aspect of Medicare, Part B, that usually gets deducted from Social Security payments. While Medicare Part A is generally no cost, Part B is what covers things like office visits and it’s partly paid through premiums based on income.

It’s the same rate for everybody up through $170,000 in income for a married couple filing jointly. Because the government wouldn’t know what a taxpayer earned until the return is filed, it’s 2015 income that determined the premiums for 2017. This time lag is one reason beneficiaries felt blindsided.

“I’d say it’s fairly complicated,” said Jamie Fritschel of financial planning and investment advisory firm Windsor Financial Group in Minneapolis, holding a client’s letter from Social Security.

Medicare tries to shield most beneficiaries from cost increases, Fritschel said. Clients who see a premium increase nearly always have reported a big capital gain, and the premium should drop again the following year. He added that his firm works hard to include expected Medicare costs in its planning, trying to make sure no clients get a letter one day they don’t expect or can’t understand.

If shareholders look to Medtronic for an explanation, they will still find Medtronic’s 20-page frequently-asked-questions summary of the inversion deal on its website. Medtronic deserves credit for the plain-language ­clarity of these questions and answers. Yet none of the questions were whether a big capital gain triggered by the deal could lead to much higher Medicare premiums down the road.

That might be because it’s only an occasionally asked question — and maybe because no shareholder would like the answer.