I wonder how they prove a mistake. It seems to me that this fare for instance isn't immediately obvious to a consumer that is a mistake, it was only in the lowest booking class (expedia showed a limited number of tickets available at that price), and is comparable in price to flights within the last two years (while fuel costs haven't risen significantly in that period).
Do they only need to prove it was a mistake on their end, or that it was a mistake and the consumer should've known if they've done research on the flights (i.e. if a flight is entered in with the last two digits transposed to give $90 off, is that a mistake)?