After finishing the AWA section, I took the 10-minute break along with 10 or so other test takers that began the test at the same time. After about 8 minutes I went back to begin the quant section only to find there was a line of 8 people waiting to check back in. The finger print machine had trouble reading the finger prints of a few people and there was only one proctor helping us, so the upshot is that by the time I got back to the computer, roughly 5 minutes of my test time had already passed!!

I did the best I could but really struggled to catch up on time, eventually allowing myself to commit the ridiculous blunder of not answering the last question before time ran out!! Verbal section was a breeze.

I ended up with a 690.
Q 42 (63%)
V 42 (95%)

My practice test results were as follows:
GMAT Prep 1 - 720
GMAT Prep 2 - 730
GMAT Prep 1 (retake) - 750

I did feel the math on the actual test was definitely harder than on the Prep tests, but I'm pretty confident I would have done much better had I not been so frazzled by the loss of time.

I'm applying to Columbia and NYU. I feel I've got a fairly strong application apart from my GMAT score. Should I retake the exam? I blame myself for not having given myself more time to check back in, but I also feel I got kind of screwed by the whole thing.

Does anybody know how my score would've been different if I had managed to fill in even an incorrect response for that last question on the quant section?

Thanks for your thoughts.