It's really fascinating (horrifying?) that some of these ETFs went to zero for a moment. I can't understand how they cancel all these trades though. Seems like the biggest accounting nightmare ever.
My take away from my experience in that industry was that you can't actually trust banks or other financial institutions to follow the rules; the best thing you can do is to make sure you're lost in the crowd. Don't stand out, and you won't get hammered down. These trades and this whole episode stood out, so of course they're going to retroactively change it to the detriment of anyone for whom it was a good thing.