I just wrote gmat prep and tallied the questions I got wrong that I was entirely capable of getting right.
I will say a large proportion of these was simply because I did not pay enough attention to the question. Because we are generally rushed on time, we (or at least I do) try to skim the questions to get straight to the working the problem. When you do this, it is easy to miss little things that make the question a lot easier, ie: restraints such as integers, key words... one of the questions even went so far as to underline the word "different" numbers, and I somehow still missed that! So my bit of advice for what it's worth is to read the question carefully!