Pursuant to Board Rule 6000.6(i), any applicant who has withdrawn from or failed to appear for two or more bar examinations must apply to the Board for permission to re-apply before taking another bar examination. Applications to re-apply should be received in the Board's office two weeks before the filing deadline of the exam you wish to take. The filing deadlines cannot be waived or extended (see Board Rule 6000.10[b]).

Applications must be in the form of an affidavit which means that you must swear under oath and under penalty of perjury that its contents are truthful and accurate, and your statement must be witnessed by a notary. Applications must include:

(1) Your name, age, residence address, email address and phone number;

(2) The facts which caused you to withdraw from or to fail to appear for each and every bar examination, not just the most recent bar exam; and

(3) The facts which support your request to re-apply including a discussion of what steps you have taken - or what has changed in your circumstances - to make it possible for you to appear for a future exam.

Applications should include supporting documentation, wherever possible, which confirms the reasons why you withdrew or failed to appear for each and every exam. Examples include medical documentation, police reports, death notices, employment letters and the like. Please note that waiver petitions under Rule 6000.6(i) will generally only be granted once absent extremely compelling circumstances.

http://www.nybarexam.org/faq/faq.htm#withdraw

Those are the rules in New York. It might be different in other states. As I read these rules, it sounds like nothing really happens for your first no-show other than you lose your application fee. But if you miss a second exam (even if you withdraw in advance) you can't take the bar again unless you can convince the state bar that you had a very good reason for missing the bar exam twice. It doesn't seem worth it to me, but it's your life.

(http://www.autoadmit.com/thread.php?thread_id=2630412&forum_id=2#26018836)