4. Tapping your retirement for emergencies. Sometimes crises arise and your emergency fund isn’t enough to cover the expenses. It can be tempting to look at all the lovely cash sitting in your retirement accounts and think about how it could solve your crisis. But not only is this a dangerous precedent to start—you never want to view your retirement as a cash cow you can tap anytime you want—but you’ll also be hit with any number of penalties for using the money early and for anything other than retirement. Then there are the taxes you’ll pay on top of that. You could easily lose half or more by taking money out prematurely. Pretend that money isn’t there until you need it, and find other ways to deal with financial emergencies now.

5. Underestimating the destructive power of inflation . Most people understand that inflation wreaks havoc on any budget, but few people plan for it. Just look at those around you who’ve retired. Have you noticed that many of them start out living pretty comfortably and then they...