Just confirmed this based on a tip from a computer engineer friend.
Due to the hard fork and Poloniex creating an Ethereum Classic (ETC; non-hard-forked chain) market, apparently a loophole in how the transactions from Coinbase resulted.
My guess is that the multi-sig wallet used by Coinbase is not set up to block out ETC chain value. It appears to be the only one to do so.
I've earned about 0.6 BTC after selling my ETC via this trick.
Analysis: The likely result of this will be everyone searching every ETH balance they have for tokens that are able to be converted into both ETH and ETC. This will result in a rapid sell-off of whatever can be sold until the ETC rewards are no longer profitable for that chain.
Update (23:48 EST 24 JUL) : Coinbase has frozen my most recent outgoing transaction.
It's been almost three hours and I sent a customer service request about an hour ago. No real response yet, except an automated.
Not fun having tx frozen :P
UPDATE 01:18 EST: Oldest frozen tx went through; did not register any ETC on the Poloniex side anymore. Coinbase must have changed the smart contract language for the multi-sig send.
This is all speculation from a semi-technical, so if I'm wrong please correct me.
Would be very curious to know how ETC will sustain itself if no one knows who owns what on the ETC chain?
How will miners remain profitable if the ROI does not surpass the opportunity cost of simply mining on the ETH chain?
How are we going to afford all of this popcorn?