https://www.coindesk.com/margin-lenders-lost-13-5-million-in-may-to-poloniex-crypto-crash
I don't recall this ever happening on Bitfinex, though we came close a couple times (and got bailed out (or saved by the price recovering) once I think by someone who bought the bitcoins during a flash crash or after it?). This was the big risk of lending out USD on Bitfinex especially in the early years like 2013.
Damn, that's a lot of coins. Is Poloniex really not paying the lenders back!? Do they not have an insurance fund? This is why futures exchanges always have insurance funds, to cover large liquidations when the book can't handle it.
I always felt like most altcoin markets lacked the liquidity for significant margin. I guess this sort of proves the point.
In other news, Binance is adding margin trading soon.