This is why you always leave really low buy offers in on a few exchanges. Flash crash was very real, anyone remember when ethereum flash crashed to 10 cents from 300 bucks?
Would they have to honour those buys at those prices?

They should and if they don't they are BS IMO..
Whoever hit market sell and dumped is the one that looses the coins, not the exchange, unless the exchange is rigged..
I have caught a couple DASH crashes and have seen a lot more.. I saw a bot go crazy once and market buy a shitcoin up like %10,000 too so it does happen both ways..
This is one reason why I almost never use market orders besides it seems they screw me anyway..
If you catch a crazy order like that I would suggest withdrawing your funds from that exchange immediately so they don't get the chance to roll the trades back on you..