This question is perhaps technically more difficult to answer than it appears, because in many cases the pigs simply change the Commandments to suit their purposes. Nowhere is this more evident than at the end of the book, when the Seven Commandments have been distilled down to one chilling statement:
ALL ANIMALS ARE EQUAL BUT SOME ANIMALS ARE MORE EQUAL THAN OTHERS.
One clear example of a violation of the Commandments is Napoleon's increasing friendship with humans, particularly Mr. Pilkington, in the final two chapters. This is a gross violation of the First Commandment, which states that "Whatever goes upon two legs is an enemy." On the other hand, the pigs are beginning to walk about on two legs by this point as well, and have trained the sheep to bleat out "Four legs good! Two legs better!"
Another example is the bloody purge that takes place in Chapter Seven, when Napoleon has his dogs publicly destroy three young pigs, several hens, a few sheep and a goose for alleged treason. This, obviously, violated the Sixth Commandment, which mandated that "No animal shall kill any other animal."
Finally, the pigs take to wearing clothing in the final two chapters, ignoring the Third Commandment. Again, most of the commandments are altered or deleted throughout the book, as when the Sixth Commandment, after the mass executions, is changed to read:
"No animal shall kill any other animal WITHOUT CAUSE."