Have you ever had a salesperson promise you something and then give you less than expected? The worst mistake people can make is over promise and under deliver. It is a sure sign to disappointment. Under promising and over delivering is beneficial, not only in sales, but also in just about every other aspect of your career.
I have seen managers over promise and under delivery. When they do make promises they will have a difficult time keeping they push their employees to fill the gaps of their mistakes creating artificial crisis. As these crisis continue the employee begin to feel less than satisfied with their work.
Beyond personnel issues is the problem of meeting those deliverables under artificial time frames. The more you promise things you can't deliver, the more risk you have of not meeting those objectives. This will reflect poorly on you and the consistency of your output.
Work quality also consistently becomes weaker over times. Trying to complete projects in short pace also encourages shortcut taking and forgetfulness of details. The quality of the work and the satisfaction of the customer is based in those details and offering something others don't have.
Whether you are working in sales, a project, or even with your families it is important to under promise and over deliver. Beat people's expectations every time and you will find that they will not only trust what you have to say but they will also believe you are more competent then you might actual be. Your reputation as a person who can deliver high quality work rises.