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The OP_HASH256 puzzle transaction described here was mined in block 211997. But I read that only standard transactions (with outputs one of P2PK, P2PKH, bare multisig, P2SH, OP_RETURN) get relayed. So how did this transaction get relayed and mined?

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"Relayed" and "mined" are completely separate processes and should not be conflated.

The transaction is valid, so it was perfectly fine to mine it (i.e. include it in a valid block).

"Standard" is a more restrictive requirement, and as you say, this transaction is not standard. That means that a default version of Bitcoin Core will not relay it, i.e. will not transmit it to other nodes on the Bitcoin peer-to-peer network. But if the transaction reaches a miner some other way, they are perfectly free to include it in a block.

We can only speculate as to how that occurred in this case, but here are some possibilities:

  • Some people were running a modified version of the Bitcoin client (or alternative Bitcoin software) that does relay non-standard transactions.

  • The creator of the transaction sent it directly to a miner, by some means other than the peer-to-peer network. Some miners or pools have web sites where anyone can submit a transaction.

  • The transaction was created by the miner herself.

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  • Thanks for the fast answer! I had in fact assumed that "valid" and "fit to relay" were the same thing. And good to know about various ways to get a non-standard transaction mined.
    – synthetic
    Commented Aug 14, 2016 at 19:56

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