What is the "1 RETURN" bug?
Have a look at the early sources here: http://www.bitcointrading.com/forum/bitcoin-clients/original-bitcoin-source-code-archives/

there is a small piece of code in script.cpp:

    case OP_RETURN:
    {
        pc = pend;
    }
    break;


which allows to spend ANY UTXO with just a very simple scriptSig OP_1 OP_RETURN