One of our students just took the GMAT at home, and passed on the following advice (I added the emphasis in bold):
One tip I wanted to share since it almost happened to me and I've been seeing it on the web...in the beginning after running the system check, taking my pics, etc. (I had even popped out the whiteboard and drew my grid for quant - which you can do before the exam begins since the whiteboard is there the whole time), the screen went blank and I got an error message. I chatted the proctor who said she was calling me, but my phone was in the other room (of course). Before I got up to answer it I asked if I should, and she said no you cannot leave your desk. If I had gotten up to get the phone as she instructed, my exam would've been invalidated! Anyway, restarting the computer and testing program resolved it, so that was a relief.
