Some transacions keep unconfirmed forever and finally they are purged by Bitcoin nodes, mostly due to the lack of fees.

Example:

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Alice is selling a pizza to Bob, Bob is now making the payment with Bitcoin.

The main goal of this attack is to store a unconfirmed transaction send by Bob for a few days (it will not be included in the blockchain because it has no fee or due to other reason), Bob might resend the payment or might just cancel the deal with Alice.

Bob forgets about that failed trade but a couple of days later, Alice, who has stored the signed transacion, relays the transaction to the network (or mines it directly with his own hashpower).

Bob does not know what is happening, he believed that that transaction was "canceled forever", he even does not remember the failed pizza deal.

Alice has now the bitcoins and Bob does not know what happened with his money.

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This might also work with the Payment Protocol because when using it Bob does not relay the transaction to the network, its Alices job to do it, Alice stores it and tells Bob to resend the payment, Bob creates another transaction (If has the same inputs as the first TX this does not work) (this one is relayed by Alice to the network).

Alice comes back a couple of days later and mines with his hashrate the first transaction (the one she didnt relayed to the network).

Alice now has two payments, Bob does not know what happened.

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I hope that I explained well this possible attack, I dont know if there is already a fix for this problem or if it is simply impossible to execute this kind of attack.

Thanks for your time.