Just before this precipitous drop, they were performing maintenance on the exchange, which was announced.
As IAmNomad reports on Twitter:
The maintenance on GDAX was announced to occur on April 12, as indicated by their status page:
BitMEX came out with an announcement, after their perpetual swap had quite an intense drop on the GDAX glitch:
At 23:02 UTC on 15 April 2017, one constituent of our .BXBT Index, GDAX, reported a trade print of $0.06 / XBT. This fed into the .BXBT Index and caused the price to temporarily move down to $888.48 / XBT which led to a number of users having their positions liquidated.
This was not a BitMEX engine or pricing issue. However, we strive to create a fair platform where users are not unfairly disadvantaged due to an error on another exchange, even if this error was an official price. As such, BitMEX will be refunding those users who were unfairly liquidated due to the pricing discrepancy from GDAX out of our own company funds.
Those users who had their positions liquidated will see the loss between $1183.00 / XBT and their liquidation price transferred back to their BitMEX Bitcoin wallet. Positions lost due to liquidation will not be reinstated.
The Team at BitMEX
We contacted other futures exchanges and there were apparently no other major effects of the glitch. Coinpit has even come out to say that despite having GDAX in their index, they utilize a "median" index, which ensures that outliers in either direction are eliminated from the computation.
This demonstrates the dangers of futures exchanges using "average" prices for their index, as if one, or two of them are glitching out, then it will drastically bring down the market index-wise, even if the quotes are fake or very temporary.