Not using all the information provided
This falls into two categories:
Information in the problem that I didn't recognize.
Information not in the problem that I assumed.
When I review the questions I've missed, I've found an inordinate number of problems that I've answered incorrectly because I failed to read the question properly.
For example, the question might refer to 'a set of positive consecutive integers', and I'll solve the program on the basis that it's 'a set of consecutive integers' or 'a set of positive integers'.
Or in other situations, I might have assumed that X and Y are integers, when really the question made no such claim.
This type of information is VERY important to solving the problem correctly. If the question tells you that the integers are consecutive, you can be sure that the answer will DEPEND on the integers being consecutive. This applies to every piece of information the question gives you.
When reading each quantitative question, you should be very conscious about EXACTLY what information the problem has given you. Occasionally the problem gives you information that might not be relevant, after reading the question you should be able to identify all the relevant information, and if applicable, any irrelevant information.
These missed questions fall under the 'stupid mistakes' category, IE the mistakes that you'll be kicking yourself for missing. Devoting just a slight amount of extra concentration while reading each question can help you avoid as many of these as possible.