Reasons for Condom failure

Condoms can fail to prevent pregnancy when they are:
Not used correctly
Used inconsistently
Broken during sex
Manufactured improperly
Damaged after manufacture Expired

Approximately 2 - 5 percent of condoms tear during use. The majority of these failures are caused by human error, which can include not using enough lube and creating microscopic tears with rings or long, sharp, or jagged fingernails, among other possibilities. Incorrect use includes unrolling a condom backwards, not unrolling the condom to the base of the penis, not leaving a half-inch of empty space at the tip of the condom, and not holding the rim of the condom down along the base of the penis when removing the penis after ejaculation. Inconsistent condom use means not using a condom every time you have sex, or not putting the condom on soon enough -- such as right before ejaculation instead of at the beginning of intercourse -- before the penis comes in contact with your partner's genitals. Any risk of pregnancy resulting from pre-cum on the fingers being transferred to a condom is unlikely. To be extra safe, a man could put on a condom at the very beginning of sexual play, rather than wait until you're ready for penetration. Condom failure may also be more likely if either of you have pierced genitals. If condoms break when used correctly, the tear is most likely due to hidden weaknesses in the rubber. These weaknesses may get past manufacturing regulation as some tests administered for strength and leaks are used to spot check a batch of condoms as opposed to testing each individual condom.