Does birth control protect entirely against pregnancy?

Preventing pregnancy. No method of birth control is 100% effective against pregnancy. The oral contraceptives are the most effective as are hormonal implants.
No. In the most perfect of circumstances, birth control still can fail (but the "best" failure rate is approximately 3 per 1000 women taking the pill). Even today, there are factors that affect birth control reliability, such as obesity, drug interactions, absorption, dose of pill, etc. The number one cause of birth control failure? --- missing a pill or taking pills late!