(The opinions expressed here are those of the author, a columnist for Reuters.)

By Mark Miller

CHICAGO -- You're getting ready to retire, and need to navigate the transition from your employer's health insurance plan to Medicare. Where do you go for help? If you're guessing your human resources department, you may be guessing wrong.

Transitioning to Medicare from your employer's health insurance presents some complex challenges -- and missteps can result in costly premium penalties and coverage gaps. But consumer experts who provide Medicare enrollment assistance say employers don't always provide the right answers.

"We've seen a real diminution of expertise among employers on this," says Joe Baker, president of the Medicare Rights Center, a non-profit consumer education and advocacy group.

Misinformation from employers is one of the most frequent sources of problems that prompt people to call the center's free national help line.

"We do see employers getting things wrong," says Paula Muschler, operations manager of the Allsup Medicare Advisor, a fee-based Medicare plan selection service. "Employers are experts on their own benefit packages, but Medicare isn't one of their benefits. People need to do their own research, and be their own advocates."

Enrollment at retirement age is hardly the only consumer problem facing Medicare beneficiaries. A new Medicare Rights Center report analyzing its inbound call center activity found that denial-of-coverage issues constitute the biggest problem (33 percent of all calls). But transition problems came in second -- 23 percent of all calls. Low-income seniors facing problems with Medicare's affordability were third, at 21 percent.

The transition from work can present some thorny, confusing issues. Here are three pitfalls to avoid when moving from your employer's plan to Medicare:

-- Missing the initial enrollment deadline. Medicare requires that you sign up in a seven-month window before and after your 65th birthday, unless you have employer coverage through active employment of your own or a spouse's elsewhere.