I talked to someone today who over-promises AND over-delivers - a recipe for getting completely overwhelmed. He is sick now with a bad cold, because he was doing more than he could handle.
It doesn't really matter why he did that; what matters is whether he does it again. Looking at this situation, he can see that he wanted to prove himself to a new client and was relatively inexperienced in both estimating a job and negotiating a rate. So this was a learning experience.
Like him, I learned that I can overestimate the amount of time I think a project will take. Projects usually take 2 to 3 times longer than I initially think they will take. So I take my original estimate and double or triple it, depending on the project and what other things are going on. It's better to deliver early or on-time than late. And it's definitely better for me to get enough sleep than have to stay up until the wee hours finishing a project due the next day. Time is one area where it really works to "underpromise and overdeliver." I used this idea when I was studying on-line for my MBA, and now for any writing I undertake (with myself as the client!).
Sometimes it's difficult to "underpromise" because a client's or employer's expectations are unrealistically high and they are inflexible. This is a set-up for resentment at best and failure at worst. I've had the experience of working so hard for so little money that I never want to work for the client again. However, I see that it was my responsibility to establish my limits, or chalk it up to "on the job education" because I made the choice freely, if ill-informedly. I learned that there may be times when I have to pass on a project or job because the expectations are too high and/or the compensation too low.
For example, when I was looking for an Executive Director job a couple years ago, I did not consider those that I believed paid too little for the effort I knew would be involved. Having been an Executive Director, I know full well how completely enveloping that job becomes, and I knew, too, what my skills and abilities were worth in the marketplace. There was a fantastic job in California, that paid about 50% less than I felt the job warranted. I told them that the salary was a problem, even though I knew it might take me out of the running (and it did!). If they weren't able to raise the compensation level to meet me even halfway, then it wasn't the right position for me. I was willing to hold out for the right job
with the right compensation.
Similarly, the terms of a consulting contract have to work for both parties. If the contracting entity can pay only X amount, then the consultant can deliver only X product. If the client wants more, they will have to pay more to get it. Otherwise, the client must figure out a way to do more in-house and farm out only a portion of the project to the consultant. The consultant can help with this task.
Of course, there are always going to be choices and trade-offs. If the client can pay only a portion of the real cost, the consultant can decide to do the entire job as long as the client is super flexible on the due date. If the consultant wants to establish him or herself in the field, s/he can take on a low-paying job with the explicit understanding that the client is getting a deal and should not tell anyone else the price. In this way, the consultant is over-delivering in the sense of giving more than is reasonable given the compensation involved.
I suppose the concept of "under-promising and over-delivering" has most to do with calculating how much time and effort will have to go into producing something, and whether the compensation or payoff is worth that time and effort. Let me be generous with myself and surprise the client.