Why can a vasectomy fail?

Due to reconnection. The likelihood to have post-vasectomy re-canalization is around 1-5 %; it usually occurs within 6 months after vasectomy, and is caused by microscopic leaks from both cut ends. The lining of vas will then grow and extend from such leaking spots result in its wandering healing process like that to cover the raw surface of an opening of ear-piercing, and finally both ends blindly meet to reconnect.
Recanulation. Sometimes, a patient can recanulize, meaning a tract forms around the procedure. We don't know why this occurs. This is a rare occurrence, but if you've performed this procedure enough, you've seen it happen. There is no 100% sure way of permanent sterilization, unfortunately.
Two types of failure. "immediate failure" describes the patient that never gets to zero sperm in the semen analysis (azoospermia). This is simply surgeon error, meaning the tube was not correctly identified and treated. "delayed failure" describes the patient that achieves azoospermia, but then is surprised by a pregnancy. This is due to "delayed recanalization, " in wihch the cut ends somehow find each other again.
Sex Too Soon. Failure rate of vasectomy (or unwanted pregnancy): failure within first year is 0.15% (1 in 400), long-term failure rate is 0.04% (1 in 2300). The most likely reason for pregnancy after vasectomy is having unprotected sex too soon. Successful vasectomy occurs around 10 weeks post vasectomy, but you should not have unprotected sex without semen analysis confirmation.