there have been reports of false negatives (people getting to the cc page but finding out they actually passed) but I have not ever heard of a false positive (getting the pop-up but failed) just make sure the status is marked as "delivery successful".
I dont know anyone who did this test that have got false neg or pos. it has been accurate. A friend of mine who took NCLEX during the summer did the PVT and it let her thru to the cc page. and within 3 days she recieved a letter she failed. A classmate of mine took her NCLEX the same day as me (the 15th). we both did the PVT on that day and i got the good pop up, she got thru to the cc page. Today she received her letter that she failed! I have received nothing... So for me, its accurate! Good luck!