Yes, a vasectomy in some rare cases can reverse itself, an occurrence known as "recanalization." In a 2005 study of more than 43,000 vasectomies, only 0.4% (183) had a "late failure," meaning recanalization occurred after post-operative tests had initially shown the vasectomy was successful.
There are some cases wherein vasectomy reverses itself after years, but this rarely occurs. In general, this procedure offers permanent sterilization for men. Here's the link for more information.http://www.vasectomy-information.com/faq/faq2.htm.