One issue I've run into a few times is knowing all of the math for a DS and having the right solution but not understanding exactly what it asks for. For example. How much more did John spend on Donuts than cake and Biscuits. Simple translation is D-(C+B). Is GMAT VERY specific when asking this type of question? If C and B offset D and the answer is 0, it is sufficient regardless, but if, for example, D ends up being double the value of C + B, I dont have an answer in dollars, but i have an answer in terms of relative size - he spent 2x as much.
HBS Class of 2014