A part of the reason why the Automated Clearing House system was setup was the collapse of Herstatt, which was a settlement failure that led to the global push for RTGSes.
It is not that difficult to imagine another such storm. A heavily shorted stock, where retail traders "rally" to stick to the man and buy + hold on to the majority of the stock in circulation. And then the shares left in pools + the open market aren't enough to cover everyone's position. Creating a cascading failure.
