GMAT Strategy

I appreciate you reading my post. I have a big decision to make and I am going to take this forums advice. Today I took the GMAT and scored 640 Q44 V35(this is the lowest I have ever scored in verbal). The issue is that I have never scored this low on a practice exam. What happened is I thought I was on the last quant question when time was up, but in reality I was on question 36(So time expired as I clicked the next button and another question popped up). This not only lowered my quant score but because I was so pissed off my verbal score had to be effected. All I could think about was how the first thing you are taught when studying for this exam was to finish in the 75 mins that were allowed(all the books say there is a significant penalty for not finishing). Due to this mistake I also considered not showing my score at the end.

My question is how effected could my quantitative and overall score be due to this mindless error? Do you suggest I retake the GMAT(my target school is University of Michigan)? I scored a 730 Q50 V38 the day before the exam. Thank you for the advice.