Underestimating what you need can cause problems ranging from having to go through the whole time consuming fund raising process again, to having to shut down the company because funds have run dry. Having to go back to the original investors and ask for more money often undermines the entrepreneur's credibility with the investors and can cause a significant dilution in the founder's ownership.
Obtaining more than enough capital may seem like a blessing at first, but it can breed a lax attitude toward expense control. "If you have it, spend it," is not an advisable motto for a new company. If the investment takes the form of equity, raising too much money means that the founder's share of the business was reduced more than was necessary--and this violates one of the maxims of entrepreneurship: hold on to those equity points!
Typical advice given to entrepreneurs is to do a cash flow projection, or cash budget, and then add 10%, 20% or even 50% to this amount, for "contingencies." These contingencies are all the things that can go wrong in a start-up venture, all the unfavorable events that can negatively affect results.
Contingency planning is a skill that does not come easily to all entrepreneurs--even those with a finance background. How do you get the cockeyed optimist (what you absolutely must be to even conceive of the idea of the starting a company), who expects the best, to plan for the worst?