Looking for the most foolproof form of birth control, many men turn to vasectomy, an effective and permanent form of contraception. Only problem is that it does not work immediately. Although the vas deferens, the tube that carries sperm from production to insemination, is cut, residual sperm in the system are still present, and can still impregnate. Here is a piece from the Self site that looks at this situation.
Shortly after publication, this piece was picked up by Fox News on their site: