The only thing ETS says is this:
If you have a complaint about the test or the test center, send your written complaint to Educational Testing ServiceÂ® (ETSÂ®), immediately following the test administration.
Address your letter to "Attention: GMAT Complaints" and submit it by e-mail, fax, or mail:
GMATâ€”Attention GMAT Complaints
Educational Testing Service
P.O. Box 6103
Princeton, NJ 08541-6103
I believe that if such a thing happens you can cancell the test and get a refund. Then you can schedule an other test.
ETS says that
If technical problems at the test center necessitate canceling your test session or prevent reporting of your scores, you will be offered the opportunity to schedule another test appointment free of charge. Or, you may receive a full refund of the original test fee.
Even though technical problems are generally not the fault of ETS, you may also seek reimbursement for reasonable and documented expenses associated with traveling to the test center. All reimbursements will be made in U.S. dollars.
To request reimbursement, please contact ETS-GMAT Customer Service with the following information within 30 days of your original appointment:
date of birth
daytime telephone number
original test date and appointment number
a brief description of what occurred at the test center
do this as soon as you encounter the problem ...
hope this helps