Aristotle addresses a number of the criticisms that can be leveled against poetry. First among these is the accusation that the events depicted are impossible. This criticism can fall under two categories. Less grave describes the event if the impossibility arises from a lack of technical knowledge on the part of the poet. For instance, he may describe a horse galloping with both front legs thrown forward, not realizing that horses do not move like this. More grave describes the situation if the impossibility arises from the poet's inability to give an accurate description of something he knows quite well.
Aristotle answers that, often, impossible events—such as Homer's description of Achilles' pursuit of Hector in the Iliad —serve to heighten the astonishment and excitement of the story. When the poet can achieve similar effects while staying within the realm of possibility, however, this route should be preferred. Aristotle lays out the general principle that a poet should always aim for a convincing impossibility in favor of an unconvincing possibility.
Further, not all poetry is meant to describe things as they are. Some poets describe things as they ought to be, and others write to accord themselves with popular opinion rather than realism. For instance, Sophocles claimed that while Euripides portrayed people as they are, he portrayed them as they ought to be. Other poets stay true to popular myths rather than realism when depicting the gods.
As for events that are not impossible but merely improbable, the poet must show either that they accord with opinion or that the events are not as improbable as they may seem.
Aristotle also discusses contradictions the poet might make in language, but this discussion is very difficult to follow without a knowledge of ancient Greek. Basically, Aristotle suggests that what may at first seem to be a contradiction in language may result from a metaphorical usage or some other poetic device.
While many errors are excusable or explainable, Aristotle asserts that the only excuse for an improbable plot or unattractive characterization is if they are necessary or are put to good use. Otherwise, they should be avoided at all costs.