What is the possiblity of falling pregnant if you forgot to take your pill the day after having intercouse?

Yes. Each day you miss a pill, or are even late taking your pill, your chances of becoming pregnant greatly increase. Most Birth Control instructions will tell you that if you miss 3 pills in a row, throw that pack away, start a new one and avoid unprotected sex for at least a month. Your chances are lower than if you had not been on the pill at all, but it is still possible.