Interestingly, Dan posted a link in which a rabbi offers a halachic analysis of whether it is permissible to exploit a fare pricing error. (Halacha, in laypersons' terms, is the study of Jewish law based upon the Torah and its descendant legal documents such as the Talmud.) Here is the discussion: http://www.vosizneias.com/111398/2012/08/08/new-york-the-elal-pricing-error-a-halachic-analysis
I noticed a not-particularly-surprising quantity of self-justification in the comments.