I got 660 today in my GMAT and I was aiming for at least 690-700.
The first issue I knew that threw me off was how random the questions on the actual GMAT can be. I sat five practice tests and covered all of the questions in the official reviews at least once and I thought I had a pretty good gauge on which topics may appear more frequently within the actual test. I was wrong.
I don't really like overlapping sets but I could definitely work through one if it came up and wasn't the 700+ questions (on which I normally can guess quite effectively) and because of this I deluded myself into believing that I would be alright as I normally only got one and around 60% of the time I managed to work through to an answer. However, as you can probably already tell, I didn't just get one question, I got four.
Four! I was horrified. First was a simple PS then a slightly harder DS (at this point I knew I was going to struggle to reach my target score) followed by two more three variable PS and DS overlapping set questions.
I believe that my exam was actually pretty good and I knew my SC would always bring my verbal down to 36-39 and unfortunately I also had an exam with more SC questions than average.
So overall I am actually really disappointed. 690 was my goal, 660 is what I got when the test was the hardest (for me) that it could be, and 700+ would have probably been my score if those four overlapping set questions had been anything else and I had received 1 less SC question.
My advice is that you have to look for your weaknesses and address every single possibility that may come up! I felt like I had truly covered all bases to achieve my goal score but all it took was for the actual test to highlight a weaker area of mine that had seemingly slipped under the radar.
And trust me that there is no worse feeling then seeing your score and instantly thinking "if I had learnt that one single question type properly I would be overjoyed."