Birth control pills are about 99% effective. So someone has to be that 1%.
Absolutly, Nothing is 100% effective. Also, if you take an anti-biotic while on the pill many times that can negate the effectivness of the pill. Or if you acidently missed a day or two.
To farther reduce you chances request your partner use a condom. A condom will also protect both you and your partner from dangerous STIs.