Off of the top of my head, before even knowing that there is a possibility of pregnancy after either a vasectomy or a tubal ligation, I have been able to think of SO MANY reasons why a couple should both go and get sterilized when they're ready. One of the most hilarious reasons I can think of came to me when I was watching "According to Jim." If you've watched enough of his shows, then you probably know which episode I am thinking of. It's the one where she thinks he has braved it, and gone ahead and gotten it snipped (such a crass way to put it). That's what she THINKS, and he lets her go one thinking that. If he never confessed, he would have had a doozy of an explanation when the next little one made his or her announcement through new morning sickness, absense of a period, what have you...If she went ahead and just got her tubes tied as well, there would be no reason to worry about whether Jim lied to her or not.
On a less humorous level, if both the man and a woman get sterilized there is a much lower risk of a possible pregnancy. If the couple was really set on not having more children, and the risk of pregnancy is already below 1% after just one partner getting sterilized, then the possibiity of a pregnancy is even lower.
Though the risk of a pregnancy is much lower after a woman has had a tubal pregnancy, there is still a slim possibility she could get pregnant after the procedure. Not only that, but tubal ligation increases the chances that the pregnancy wouldn't go full-term. It could even result in an eptopic pregnancy (in which the fertilized egg is trapped in a fallopian tube). This can be extremely painful, and if left undetected and not terminated it can even cause death.
Recent studies have shown that having tubal ligation can greatly reduce the chances of a woman getting Ovarian cancer. For this reason, I think it's a great idea for a woman to go ahead and have her tubes tied once she is ready to stop having children. With the risk of an eptopic pregnancy, if the man goes ahead and also gets sterilized this risk is close to nil.
There is always the most slim possibility that, even with both partners getting sterilized, a pregnancy occurs. At least this way, both partners have pulled out all of the stops that we can currently think of.