But this is nothing about matching undercollateralized positions, as I believe a fat-finger order is simply falling into that category by accident (I'd be quite surprised if it was by design). And it is certainly NOT an "expected pattern of behavior". But, at least if more people are aware of the problem, they will be less likely to make such a mistake.
However, let me ask you this. If by accident you entered a BUY order for 8 BitUSD at 1,000,000 BTS instead of 1,000,000 BitUSD at 8 BTS, would you feel okay with your irreversible 8 BitUSD at 1,000,000 BTS each, despite the fact they were clearly available for under 8 BTS each? Perhaps that's something to think about... I suppose the bitshares network could always pull an "ethereum-type solution" and "bust" the trade if it's sizable enough. But somehow (to me at least) that doesn't quite seem the best way to go.
Fat fingers are unforgivable. You won't expect a refund if you placed a wrong order in other exchanges.
Margin calls are only one direction, they can only be orders to sell BTS for bitUSD. Your new order to sell 1M BTS for 8 bitUSD will compete with them, but not match with them. Whether your order will get filled or not, at what price, depends on the orders on the opposite side, say, who wants to buy your BTS, and at what price.
At present there is no margin call on the book of BTS/bitUSD pair. You will get best offers no matter what price you placed.
my response above addresses all these points as well
This is a good example, but also even having some lower asks come in while you're entering your bid or something like that and still being filled at a higher price is unacceptable. Other trading platforms don't function like this, so why would you intentionally choose to utilize a platform that's designed to occasionally fuck you?