The allegorical sense allows us to see the meaning of Old Testament events in Christ. For more information on this see Mark Shea's Making Sense Out of Scripture. Since the Holy Spirit is the author of Scripture the message about our salvation is without error. In practice this means that apparent contradictions are not due to errors in God's word, but rather due to translation mistakes, deficiencies in our knowledge or faulty interpretations. This does not, however, mean that Scripture could not contain errors of fact in history or biology, for example, arising from the limitations of the human author, but rather that their are no errors in the inspired truth which the Scriptures contains with regard to our salvation.
Good commentaries help clear up gray areas or poor translations.