OK, here is a little sex-ed lesson:
1) Without any form of contraception any single act of intercourse has ~15% chance of fertilization.
2) Of fertilized eggs, 3/10s spontaneously abort during the first month due to malformation, improper implantation, environmental problems, etc
3) On BCPs, the egg is almost never (<5%) released from the ovum.
4) In the small minority of cases, when the egg does release, it cannot be fertilized due to a thickened cervical plug.
5) Adding in condom use (97-99% effective)
Exceptional rogue egg is released from the ovum of a BCP using woman. This egg waits in the fallopian tube and hopes to be one of the exceptional 15%. Assuming intercourse in the perfect 3-5 days of the month the egg is even viable for impregnation, with less than 1000 sperm escaping the condom, even if not every single sperm dies from the spermicidal pH levels present outside the cervix, the survivors will be stopped by the cervical mucus plug. Assuming a few magic sperm manage to miraculously breach the cervix, they still must properly navigate the uterus, and if they successfully do that, they still have a 50% chance of entering the wrong fallopian tube or dying in the tube before reaching the egg.
Assuming, after violating greater odds than winning a jackpot lottery 20 times in a row, one little swimmer manages to reach and fertilize that rogue egg, do you really think slightly less than optimal uterine conditions will deter it?
To address the anecdotes of "I had a friend on the pill who got pregnant...", let me say that, given the inefficient reproductive process of humans, I can say, statistically soundly, that they were not using the pill properly.