Anytime you miss a pill, you should double up the next day. If you miss a pill and just disregard it altogehter (for that day) you do have a chance of getting pregnant.
The pill works by suppressing ovulation, but if you miss a pill you're not going to just suddenly ovulate that afternoon.
ive been oni birth control since who knows when and ive missed a day here and there, its not recommended but nothing to serious should happen