First of all, take a deep breath!

Second, a drop like that is not uncommon at all. In fact, I just finished working with a student from one of my Chicago classes who dropped 70 points on her penultimate practice test a week before the real exam. Her actual score turned out 20 points higher than her best practice exam.

There are many possible explanations. Perhaps you had a bad day. Maybe the mix of questions hit your weaknesses while avoiding your strengths. Maybe you made some uncharacteristic mistakes. Whatever the reason, it's important that you review the questions you missed to see if there's something you can identify.

I would treat the drop as an anomaly unless your next test score does not return to where you expect.

Hey-

This actually just happened to me as well. I was scoring 670's on manhattan gmat 47 quant 35 verbal and the 4th one I got a 42 quant 32 verbal. I didn't go down that much I just had a bad day and not sure what exactly happened. I see that these tests have massive fluctuations and don't get bugged out by it- just forget about it and move on.