7 surprising expenses people don't expect when they retire, according to experts

Roberts said she reminds clients that Medicare is not free.

"Because so many of them are unaware that there are both premiums and cost-sharing associated with Medicare, they fail to save enough for it," she said.

Only parts of Medicare are free, and most Medicare recipients pay a monthly premium that can increase for those with high incomes. Prescription drug coverage, known as Medicare Part D, also has a monthly premium depending on which plan you choose.

These costs can add up for people on fixed incomes who aren't planning on it.

Another unexpected expense? Supplemental insurance.

That's because Medicare doesn't cover all of your healthcare needs.

"Since Medicare only covers 80%, many Americans are surprised how much healthcare costs still are when they have to cough up funds to pay for the remaining 20% of costs for their medical care," Roberts said.

Read more: 8 tips on mortgages for first-time home buyers, according to an expert

For many, the 20% comes in the form of supplemental insurance, often called Medigap plans. These plans help take care of the costs not covered by Medicare, including copays and some additional doctors' fees.

No one likes to imagine a day when they might not be able to care for themselves.

But for many, spending your later years in an assisted care facility or nursing home is a reality.

Patti Black, a financial planner at Bridgeworth Financial, stresses to her clients that they need to prepare for this potential reality. But she also highlights a way to potentially help defray some of these costs: taking care of your health now. She advises clients to focus on a healthy lifestyle that includes physical activity.

"Your health also determines whether you will be able to continue living independently," she told Business Insider.

One unusual retirement complication that has popped up among Boomers and the oldest members of Gen X is an increase in divorces.

A 2017 report from Pew Research Center found that that "gray divorce," as it's known, has doubled since the 1990s, while its declined across almost all other age groups.

"It is most often the wife who asks for divorce after age 50," Black said.

Divorce is stressful at any time. But when it comes on the heels of retirement, it can leave you feeling less financially secure or in a protracted legal battle over your savings and investments.

"You have to be patient with yourself and with your spouse or you may find yourself part of the gray divorce phenomenon," Black said.

The trend of boomerang kids — young adults who move back in with parents after college — has been a concern for many parents who are on the cusp of retirement for years.

But Michael W. Landsberg, a financial planner and accountant at Landsberg Bennett Private Wealth Management, encourages his clients to think about their aging parents too.

"Caring for elderly parents has caught a few clients off guard," he said. "There is a knowledge gap between what the parents have and what the kids think their parents have. So, often times, they have to dig into their pockets to make up the difference."

Landsberg notes that it's critical to have these conversations with your parents so you're not faced with any major surprises that could put a dent in your retirement plans.

Very few people spend much time thinking about inflation rates. But Taylor Schulte, a financial planner and host of the "Stay Wealthy" podcast, said he always keeps that in mind when working with his clients.

"It's a sneaky expense that hides behind the scenes and, over long periods time, can be one of the biggest threats to an investors nest egg if not accounted for," he told Business Insider.

Inflation rates can impact the market, so Schulte advises running retirement planning scenarios with inflation in mind.

"That should give you some additional confidence needed to sit back, relax, and enjoy your newfound freedom," he said.