Antibodies and Birth Control Clash

About a week ago, I walked into a shoe store and over heard a guy talking with his co-workers. They were having a conversation about his wife taking birth control and antibotics. These two medications conflicted with one another resulting in his wife becoming pregnant although she was taking birth control. It's well known that some medications do conflict with one another, but the thought of antibiotics and birth control conflicting was very shocking to me.