Tether released a statement 21 November 2017, claiming 30 million USDT tokens has been granted to an unauthorized address. An emergent patch of omnicore is also released to fix the problem. The USDT tokens tainted by the address is blacklisted and not redeemable for USD. The market took a hit but soon recovered most of the loss. Tether ask client to be patient for further investigation. OKCoin, Huobi and Binance announced suspension on the USDT Deposit and Withdrawal shortly after.
http://news.8btc.com/30-million-usdt-hacked-might-favor-bitusd-of-bitshares