A common mistake by retirees is to underestimate their costs of Medicare, Medicare supplements, and out-of-pocket medical expenses.  For instance, it’s not uncommon for a retiree couple to pay $9,000 per year on Medicare premiums and also thousands of dollars out-of-pocket on medical bills.  And don’t forget about annual inflation.  Underestimating these expense items can ruin a good day and maybe someone’s retirement years.