Neither Wizards of the Coast nor the Tournament Organizer guarantee any specific distribution of card rarities or frequency in a particular booster pack or tournament pack. If a player receives an unconventional distribution of rarities or frequencies in a particular booster pack or tournament pack, he or she must call a judge. The final decision to replace or allow the atypical product is at the discretion of the Head Judge and the Tournament Organizer.
Players should open and verify a pack one at a time. It gets messy if someone is missing an uncommon/has an extra uncommon and they find out after all 6 packs are opened.
If a player is missing a card, the Head Judge can open a booster and pick a random card of that rarity to replace the missing one. If a player has an extra card, your options are to leave it, replace the whole pack, or have the player add a random card from the rarity of the extra one to their trade binder. It is generally best to minimize disruption to the pool by removing a card.
It is also worth noting that in the past Wizards of the Coast has rarely and (unintentionally?) put product from a future or previous set into a pack. If that occurs then this would also fall under the abnormal product policy listed here.