A company going by the name of Parity Technologies had created a multi signature ethereum wallet, further developing it to better that of the original Ethereum foundation.
Turned out their "development" had bugs and issues.
According to reports, an amateur programmer involved in this accidentally ran a kill function that led to the smart contract deleting some of its own code.
And it so has it that some of the functions that were deleted was the one that allowed Ethereum to be withdrawn from the wallet.
About 500,000 ether is locked up in Paritty Technologies wallet, which at today's rate is roughly $400m.
Despite hopes and subtle pleas, the Ethereum community has refused to fork just like it did in the case of the Stolen DOA funds. (News Flash: If Ethereum hard forks, forcibly re-insert the deleted withdrawal code and store the functionality to the multi sig wallet.)
So it is very possible that over $400m has been lost to the wind due to the technical issues of Parity technologies. Know whose Company's wallet you choose to use to avoid stories that touch in Future.