I've actually used this concept at an old job. When I was given a new project the business people always wanted it done at a particular date, but it was always an unrealistic timeframe. I'd then spend sometime thinking about how long I thought it would take me, but I would always add 2 weeks or 25% to the estimated time, which ever was larger, just to deal with the human element.

This could include changing requirements, poor communication, illness, being blocked by other changes, etc.

I learned that you can get away with giving people extended deadlines as long as you hit them.