Well I didnt fail it so I wouldn't know the exact answer from my own personal experience but I would assume you would just waste your money to pay for another registration fee and sign up for another test day. Like I said before, most fails I heard about were from making obvious and dumb mistakes on the exam that caused them to easily fail. Basically, don't Rx somebody something that they say they clearly have an allergy for, don't EXT teeth that have small occlusal caries only, do SRP if they have obvious sub-g calculus all over the crappy radiographs, ect. The exam is very basic, don't stress about it, take it and move on with your life.