TLDR: Exactly what you might have guessed: the exchanges backed their financial operations with a datastore without transactions, tragedy ensues. This can't be

TLDR: Exactly what you might have guessed: the exchanges backed their financial operations with a datastore without transactions, tragedy ensues. This can't be viewed as an indictment of "NoSQL" though; there are plenty of databases featuring ACID transactions that qualify as "NoSQL" (FoundationDB and OrientDB come to mind). That said, none of the popular NoSQL contenders have them, and the new guys with transactions tend to be commercial, so if you need transactions in an free and open-source datastore, just use PostgreSQL.