Gavin Andresen - 2015-01-07 14:54:43

Three important points to consider about OP_RETURN:
1) A transaction can only have one OP_RETURN output.  Two or more is simply invalid (not non-standard but invalid) and will be rejected by all nodes.
2) The "payload" of the OP RETURN output is limited to 40 bytes.  More than 40 bytes is invalid as well.
4) The OP_RETURN output is unspendable and thus normally should have a value of 0 BTC however be aware you can set it to any valid value and if you do so accidentally then the coins are effectively destroyed.

"Three sir!"

Actually, 1 and 2 aren't correct: the one-output and only-40-bytes checks are "what is a standard transaction" policy rules. If you can get a miner to include it in a block, a transaction with 11 100-byte OP_RETURN outputs is valid.