Waking up to find 512 btc in your account =/= stealing/thieving.
He sold those Bitcoins shortly thereafter. So yes, it is stealing.
Transactions aren't reversible by design - those Bitcoins are now his.
You can't have it both ways people - you cannot have a system with 'no chargebacks' but call the lack of a receiving party to return mistakenly sent funds 'theft'.
Any and all rsposibility, if any at this point, falls on the programmer who coded this, not the person who received the funds.
This is a stupid way to think: "Oh, this grandma dropped her purse and it landed in mine. Well tought luck, it's her fault and now my money.", "This guy didn't lock his door, well, his fault, I can walk in and take his stuff, that's the physical world".
Of course we can have it both ways: on the technical side, a transaction is final. But it can well be reversed socially by making a new transanction to give it back.
If a friend lends you a 100-dollar-bill, the physical process of him giving it to you is not reversible. Yet you give it back, unless you are an anti-social asshole.
If someone accidentially give you a 100-dollar-bill, you give it back, unless you are some anti-social asshole.
Anyone who says "tough luck" or "coders fault" here, should go back to the jungle or maybe in jail. But probably even there, some decency exists.
This is sad. What has become of this once awesome community?