Mistake fare or not, legally, one of the big questions is whether sufficient consideration has changed hands - while something like a $5 fare would almost certainly not pass muster a $900 fare (including taxes, so perhaps $650 or so?) almost certainly would.

Moreover, tons of businesses are using IT to flag unusual things - if Delta has such a feature, it is obvious that it either was ignored (which happens) or the fare was within the bounds set.