Some oil base lubricants can also breakdown the latex in the condom creating more little tears. That's why you always use water base lubricants.
Also, sometimes people forget to put the condom on first thing.
Usually, if you follow the directions
perfectly then your chances are pretty slim. So when the directions say, "Remove condom before erection is lost," then you need to so your chances of leaks or spills onto the vagina don't happen.
Thats why back up methods of birth control are always a good thing.