Originally Posted by
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I'm guessing they sometimes payout using funds just received and not fully confirmed.
So when Blockchain wallet sees the transaction has unconfirmed inputs it throws up a warning message.
It's a trick where you can send a real looking transaction to, say a book, with such a low fee that it wont be picked up by a miner very fast or at all. Then create a second transaction with the same inputs and send that to yourself with a higher fee.
When the second spend gets confirmed, the first transfer to the book gets cancelled.
It's the main reason books require confirmations before crediting. If they didn't, some people would be constantly making deposits, playing the unconfirmed funds and if they win just leave it to eventually confirm and get paid, or if they lose throw out a double spend and get their money back.