If you are referring to Arnold's fallacies regarding lit crit, the two fallacies he mentions are historical and personal.
"Arnold identifies two major fallacies people make when judging poetry. The first is that they judge according to the history of a poet or poem. People give too much credit to a poem on the grounds that it is ancient. The second fallacy is judgement based on personal emotions. We tend to judge our contemporaries too harshly because of popular belief that only ancient poetry is worthy of praise. Arnold argues that poetry should be judged on individual merit instead of historical or personal likes and dislikes."