Clients of Coinbase Pro trading platform (previously named GDAX), were not able to to place trades for BTC/USD pair beginning 08:45 PM ET on Thursday. To address the issue, Coinbase cleared the book of open orders and entered post only mode of new orders without matching though. This situation has led to various grievances from the traders. According to Coinbase Pro "The BTC-USD order book is now in full trading mode".
Source: @CoinbasePro twitter account.