Spending habits like other habits, do not change very much. A lifetime of spending habits will not suddenly change when someone retires. So people usually underestimate how much income they will need to live the life they want after they stop working. 

Be realistic, review your annual credit card summary and bank statements to determine your actual cash outflows.

Retirement income is usually generated from three different sources. Dividends, interest, and/or capital gains.

Timing is everything when it comes to retirement.

For example, let's assume a person retires needing 6% annual growth on their portfolio and luckily, at the time of their retirement, interest rates happen to be 6%. That person could then generate income with little to no risk to capital and retire comfortably.

On the other hand, if interest rates are at 3%, then this person would need to take a sizable risk to generate the extra 3% a year needed. 

People planning for retirement should know the growth rate they need and how they will generate it.

To achieve retirement goals people have to generate a return on their portfolio. So many people assume a 6% or 8% return will be there without knowing the possibilities. 

People may look and say, "Well the stock market has averaged 9% or 10% for the last 100 years," not realizing that there are periods when the stock market declines for multiple years or may not make any headway for 10 or 15 years. (review the stock markets of the 1970s)

Too little emphasis is put on how to generate CAGR (Compound Annual Growth Rate). Most people just assume that the CAGR will be there. Portfolio Management is CRITICAL.