Here is one big retirement mistake: Believing you won’t encounter any health complications for the rest of your life. Maybe you will stay healthy, but having a backup plan for if things go south can help prevent unnecessary pain and stress.
One large mistake people tend to make is not contributing enough to their retirement accounts to receive the full employer match. If you are not contributing enough to your 401(k) to receive the match, you are just giving up free money.
The biggest retirement mistake I see is people not considering all the non-financial aspects of retirement. They think they'll just stop working and spend all their time on a quasi-vacation playing golf or taking it easy.