Apparently, plenty of people on this site think that birth control failure is an excuse made up by women to cover sleeping around and/or not using anything at all. This could not be further from the truth. I researched failure rates for many methods. While most may seen like a slim risk to ONE WOMAN, the members of this site, I'm sure, add up into the tens/hundreds of thousands. So, let's play a little game. First, I'll list the methods and failure rates. Then, I will calculate the number of failure per a large group of women by the percentage. Maybe this will open some judgmental eyes...
Number of women 1,000 10,000 100,000
IUD .2 to .8 8 80 800
Injection 6 60 600 6,000
Combination pill 6 60 600 6,000
Progestin only pill 9 90 900 9,000
Patch 9 90 900 9,000
Ring 9 90 900 9,000
Diaphragm/cap 12 120 1,200 12,000
Condom 18 180 1,800 18,000
Spermacides 28 280 2,800 28,000
Fertility Awareness 24 240 2,400 24,000
Even tubal ligations and vasectomies are quoted at less than 1%, NOT 0%
Note that the most effective methods, IUDs and tubal ligations/vasectomies, are also the most difficult to obtain and the most expensive. Also note that some women cannot use any hormonal methods, and must deal with the higher failure rates of non-hormonal methods. Also note that when applied to the general population, what may seem like a small failure rate translates in real life to significant numbers.
Now, proceed with picking this apart, saying that if someone didn't really want to get pregnant that they should be using 5-7 methods all at once, or find someway to put your own spin on it.
(Figures quoted from: http://www.cdc.gov/reproductivehealth/UnintendedPregnancy/Contraception.htm )