Condom
The most common reason for a condom failing is that the user has not put it on properly.  A condom can also fail because it has been damaged, because the latex has weakened, or because it has come off the penis during sexual intercourse.  Something else that can cause a condom to not work, as it should, is sabotage. This can be when one of the partners wants a baby, but the other one does not. Usually they pierce a condom with a sharp object, making the condom useless. Some people see it as not ethical, because one of the two sex partners does it without the other one's knowledge. There are websites that show how to do it, if the woman wants a baby, but the man does not.