As of May 8, DOT will no longer require carriers to honor mistaken fares as long as the carrier (1) demonstrates that the fare was a mistake, and (2) makes the purchasing consumer whole by refunding the ticket and reimbursing the passenger for any reasonable, actual and verifiable out-of-pocket expenses made in reliance on the purchase price of the ticket (such as non-refundable hotel accommodations). Carriers are permitted to require passengers to provide proof of such costs in order to provide a refund.
The requirement to cover reimbursable costs will force carriers to move quickly to detect and notify passengers promptly of any mistaken fares published in error. This may prove challenging with online travel agencies and travel agent sales. There is also likely to be a lot of back and forth about what is (and what is not) a "reasonable" out of pocket cost and will require airlines to amend their policies to cover this change. While this is a big symbolic win for carriers, the challenge will be to find a way to implement this rule that insulates carriers from fraudulent hotel claims, fees for connecting flights, tour bookings, etc.
The May 8 policy will remain in effect until DOT publishes a permanent ruling on treatment of mistaken fares, whether in the EAPP III Final Rule or otherwise.