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 two to five 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 single time a person has sex, or not putting the condom on soon enough