I’m 72 and still working. Because they automatically signed me up at 65, I did not know I was in Medicare. Last year my employer offered the Health Savings Account (HSA) which I signed up for. 10 months in they told me that I was inelgible because I was in the Medicare system even though I was on my work healthcare system. I had to repay all the copays and other medical bills that I used the HSA for and then I had to pay the income tax on it to boot. Writing a check for $2K was hard.
The same think happened to a former coworker of mine. He was still working and had employer-sponsored health insurance. HR didn’t tell him that when he turned 65 he no longer could contribute to an HSA. I learned from his mistake, and maxed-out my HSA contributions before I turned 65. You can still draw down from an existing HRA after age 65, but you can’t put money into it.