Voucher has expiration date.
In theory it should be valid one year from buying date, but I bought my voucher on July 2008 and it was valid only to the end of April 2009.
From last 2 or 3 years Sun has "free retake offer", look here: http://www.sun.com/training/certification/packages.html If you fail, you can take second exam for free.
The retake voucher offer is only available with packages ordered in Afghanistan, Albania, Albania, Algeria, Angola, Argentina, Australia, Austria, Bahrain, Bangladesh, Belgium, Benin, Bosnia and Herzegovina, Botswana, Brazil, Bulgaria, Bulgaria, Burkina, Faso Burundi, Cameroon, Canada, Cape Verde, Central African Republic, Chad, Chile, Colombia, Comoros ,Congo, Côte d'Ivoire, Croatia, Croatia, Cyprus, Czech Republic, Democratic Republic of the Congo, Denmark, Djibouti, Egypt, Equatorial Guinea, Eritrea, Ethiopia, Finland, Former Yugoslav Republic of Macedonia, France, Gabon, Gambia, Gaza Strip, Germany, Ghana, Greece, Greece, Guinea, Guinea-Bissau, Hong Kong, Hungary, Iberia, Iran, Iraq, Israel, Israel, Italy, Jordan, Kazakhstan, Kenya, Korea, Kosovo, Kuwait, Lebanon, Lesotho, Liberia, Libya, Luxembourg, Madagascar, Malawi, Malaysia, Mali, Malta, Mauritania, Mauritius, Mayotte (France), Mexico, Montenegro, Montenegro, Morocco, Mozambique, Namibia, Nepal, Netherlands, New Zealand, Niger, Nigeria, Norway, Oman, Pakistan, Philippines, Poland ,Qatar, Republic of Macedonia, Réunion (France), Romania, Romania, Rwanda, Saint Helena (UK), São Tomé and Príncipe, Saudi Arabia, Senegal, Serbia Seychelles, Sierra Leone, Singapore, Slovakia, Slovenia, Somalia, South Africa, Sri Lanka, Sudan, Swaziland, Sweden, Switzerland, Syria, Tanzania, Thailand, Togo, Tunisia, Turkey, Uganda, United Arab Emirates, United Kingdom, United States, Venezuela, Vietnam, West Bank, Western Sahara, Yemen, Zambia, Zimbabwe. Sun employees are not eligible for this offer.
Unfortunately we can not get a free retake of exam in India.