Experts warn that the most expensive mistake retirees make isn't poor investing—it's ignoring these 11 red flags. Overlooking health decline can trigger costly medical emergencies, while neglecting estate planning risks family disputes and tax penalties. Failing to review insurance coverage leaves assets vulnerable, and ignoring tax implications of retirement accounts creates unnecessary liabilities. Underestimating longevity forces premature spending, while disregarding investment diversification invites market losses. Skipping professional advice compounds risks, and misjudging housing needs leads to expensive relocations. Ignoring inflation's impact erodes savings, and overlooking long-term care costs devastates finances. Neglecting social security strategies shortens retirement income. This oversight creates cascading financial vulnerabilities that undermine decades of planning.