To underestimate can lead to excellent results. For instance, if you underestimate your ability to solve differential equations, and therefore, study aggressively during the course, you might find yourself pleasantly surprised to receive the top score in the class.
On the other hand, the act of underestimating can also have detrimental results. For example, if you perform an enormous lab, and work all week on the report, only to realize it's true massiveness during the midnight hour the night before it's due date. Here, because the slope is always slippery, another unfortunate event might occur if you also underestimated the amount of sleep you need, and therefore, fall asleep while your playing your french horn during your lesson.
However, the negative effects of underestimating can be greatly reduced if a very kind person buys you a cup of coffee, as one of the admission councelors did for me this morning.