Few retirement decisions are as critical, or as easy to get wrong, as when and how to take your Social Security benefits. The rules can be so convoluted that many people rely on what they're told by Social Security employees, but that could prove to be an expensive mistake.

Certified financial planner Kate Gregory of Huntington Beach, Calif., uses sophisticated Social Security claiming software to recommend strategies that maximize clients' lifetime benefits. Gregory advised one of her clients, a widow, to apply for her own small retirement benefit first so that her survivor benefit could grow, then switch to the larger benefit later. When the woman contacted Social Security, however, she was told she could get the survivor benefit only.

"That left her really flustered," Gregory says.

The widow eventually was able to get the benefits she's entitled to in the correct order, but financial planners worry about people who don't get professional advice and who could be led astray.

"Most people are going to say, 'Well, that's what the government told me' and let it drop. And that's unfortunate," says CFP Mary Beth Franklin, author of "Maximizing Social Security Retirement Benefits."

The cost of mistakes

A lot of money is potentially at stake. The difference between the best claiming strategies and the worst could add up to $100,000 over the lifetime of a single person and $250,000 for married couples, says William Meyer, CEO of Social Security Solutions.

People who apply for benefits may be told they're eligible for six months of back payments and that claiming the lump sum reduces their monthly benefits only slightly. Over time, though, that reduction adds up, especially when cost-of-living increases are factored in.

"The agents are saying, 'Hey, your monthly income only goes down $50,' or whatever it is," Meyer says. "They don't tell you, 'Hey, over your lifetime, that could be a reduction of $20,000.'"

A 2018 report from Social Security's Office of the Inspector General estimated that 9,224 widows and widowers age 70 and older were underpaid by about $131.8 million because they weren't properly informed of their options.

Where to get information

Social Security employees aren't supposed to give advice, just education, Franklin says. But she and other financial planners can relate many stories of people being encouraged to sign up early when waiting was a better strategy, or receiving bad information such as being told they weren't eligible for certain benefits or that they couldn't take actions that were in fact allowed.

People can educate themselves by visiting Social Security's recently redesigned site and learning how the various benefits work, Franklin says. AARP has a free Social Security claiming calculator that allows people to model different strategies. Or you can spend $20 to $40 and up to use paid software, such as Social Security Solutions or Maximize My Social Security, that allows you to model more complicated situations. Consulting a financial planner who uses similar software also can be a smart move.

Franklin urges people to learn as much as they can before approaching Social Security, then keep a record of all interactions with the agency in case they need to appeal or correct a decision.

"I'm not here to bash Social Security representatives because most of them work very, very hard," Franklin says. "But the rules are so complex."