Effective 8 May 2015 the US DoT will no longer enforce the requirement that airlines honor mistake fares. Citing "concern regarding how quickly mistaken fares are spread though postings on aviation and travel websites, forums, and blogs," the DoT sought comment in May 2014 about changing the rules. Even with review of those comments pending (comment period closed in September 2014) the Department has chosen a policy of "non-enforcement" effective with the issuance of the statement last Friday.
Essentially the issuance of "thousands" of mistake fare tickets before the airlines could react and plug the holes was sufficient to get their collective voice heard by the DoT and stop the game.
Read more of
my analysis on the DoT stopping enforcement of mistake fare rules.
Read the official statement from the DoT.