BLOCKCHAIN Issue ! never seen this before

[cryptosun]

hello today i received a payment of 0.15 BTC around in my btc wallet , it was not confirmed , i paid the person the ecurrency he needed , in 10 mins i got a msg from blockchain.


Transaction Confirmation Failed

A transaction made through Blockchain.info has been removed from our database because it was taking a long time to be included in a block.

Any funds have been returned to your wallet and you can now spend the coins again. Please consider including a larger fee in future transactions.

xxxxe971c339247c400e0711d0dafe303636fbbxxxxxxxxxxxxxxxx
xxxxxBNVtwDmWsFkcbf1LWRVxxxxxxxxxxx
   xxxxxxb2kaFofioqfRXP7wMdxxxxxx 0.15614496 BTC


and then immediately i went to my blockchain address the funds were there and had 3 confirmations and then when i clicked on the txid which i received with the transaction failed email

I SAW THIS RED MESSAGE
Transaction rejected by our node. Reason: Transaction was previously accepted but has been pruned from our database.

PLEASE TELL ME IS IT RISKY TO PAY BEFORE THE CONFIRMATIONS TO THE CLIENT.
was worried thats why i asked here

Any help will be appericiated.

Regards

[achow101]

You should always wait for at least 1 confirmation on an incoming transaction before spending it. This protects against double spends and the current transaction malleability attack. You should also send transactions with a decent fee so that it actually gets confirmed. How much was the fee on the transaction that you sent?

[cryptosun]

You should always wait for at least 1 confirmation on an incoming transaction before spending it. This protects against double spends and the current transaction malleability attack. You should also send transactions with a decent fee so that it actually gets confirmed. How much was the fee on the transaction that you sent?

Bro i was not even sure that in blockchain transactions there could be a attack too , coz i also received many big payments with no issues , and i sent him payza and he sent me btc to my wallet , But the funds are in my account , so why i received the message payment failed?

does that mean blockchain covered it from its end? and i allways wait for 1 confirmation correct?

[jbrnt]

I SAW THIS RED MESSAGE
Transaction rejected by our node. Reason: Transaction was previously accepted but has been pruned from our database.

It is likely that the transaction was sent with a small fee, blockchain.info pruned the transaction from their database but it was later included in a block by the miners. You received a delayed email from blockchain.info when they removed the transaction from mempool, but when you check again, it has already confirmed.

Don't trust information from blockchain.info alone, always check other blockexplorers for confirmations and unconfirmed transactions.

[cryptosun]

I SAW THIS RED MESSAGE
Transaction rejected by our node. Reason: Transaction was previously accepted but has been pruned from our database.

It is likely that the transaction was sent with a small fee, blockchain.info pruned the transaction from their database but it was later included in a block by the miners. You received a delayed email from blockchain.info when they removed the transaction from mempool, but when you check again, it has already confirmed.

Don't trust information from blockchain.info alone, always check other blockexplorers for confirmations and unconfirmed transactions.

so bro is it possible that after a payment unconfirmed in our btc it can be reversed? because the other friend above said wait for 1 confirmation before sending?

[achow101]

I SAW THIS RED MESSAGE
Transaction rejected by our node. Reason: Transaction was previously accepted but has been pruned from our database.

It is likely that the transaction was sent with a small fee, blockchain.info pruned the transaction from their database but it was later included in a block by the miners. You received a delayed email from blockchain.info when they removed the transaction from mempool, but when you check again, it has already confirmed.

Don't trust information from blockchain.info alone, always check other blockexplorers for confirmations and unconfirmed transactions.

so bro is it possible that after a payment unconfirmed in our btc it can be reversed? because the other friend above said wait for 1 confirmation before sending?

It can be reversed, but with great difficulty. Reversing would essentially be double spending, e.g. there are two transactions that spend the same inputs. To reduce the risk of double spends, you should wait for a transaction to get at least 1 confirmation. That means that the transaction has been included into the blockchain and that it will be almost impossible to double spend.

[shorena]

-snip-
so bro is it possible that after a payment unconfirmed in our btc it can be reversed? because the other friend above said wait for 1 confirmation before sending?

Yes, unconfirmed transactions can be reversed. Well not actually reversed but pointed at a different address, which is essentially the same. Usually its very complicated and it would not explain why you received the (or some) coins anyway. Let me try to explain what the current problem is.

If you send a transaction it gets an ID, this TX ID is just an hash and not all parts of the transaction are actually signed by your private key. There are parts that are not signed, but hashed, thus if someone takes a transaction you created and modifies a part of it that is not signed they can change the hash. That is all they can change. They can not change where the coins come from, they can not change where the coins go to and they can not change how many coins are transfered where. Only the hash. Now, why is this important if the important parts of the transaction (the part that deals with the coins) are the same? Its important because of the way transactions work. They always refer to an old transaction. They create a long chain of transactions back to the block where the coins have been mined and created. And this reference works with the TX ID. You create such a chain and the TX you use along the way must not be confirmed. You can create a TX A that sends coins to your friend, who uses them without a confirmation (because he trusts you) to create another TX B to send some coins to his girl. Now this attacker comes in and changes your TX ID. Now there is TX A, the modified twin of A, lets call it A2 and B. B is only valid if A gets confirmed. As it happens durring this attack A2 gets confirmed and B now points to a transaction (A) that is no longer valid. A tried to use the same coins as A2 and A2 is confirmed, thus A will be removed from the memory of all nodes that know about the block that confirmed A2. Your friends girl will be pissed, but its not your fault nor your friends.

In your case I suspect that blockchain.info warned you about a TX like A, but A2 was confirmed so you got the coins anyway. They are the same coins your tradepartner send you, they just arrived under a different TX ID which caused the wallet to get confused.

[BTCdoaA]

-snip-
so bro is it possible that after a payment unconfirmed in our btc it can be reversed? because the other friend above said wait for 1 confirmation before sending?

Yes, unconfirmed transactions can be reversed. Well not actually reversed but pointed at a different address, which is essentially the same. Usually its very complicated and it would not explain why you received the (or some) coins anyway. Let me try to explain what the current problem is.

If you send a transaction it gets an ID, this TX ID is just an hash and not all parts of the transaction are actually signed by your private key. There are parts that are not signed, but hashed, thus if someone takes a transaction you created and modifies a part of it that is not signed they can change the hash. That is all they can change. They can not change where the coins come from, they can not change where the coins go to and they can not change how many coins are transfered where. Only the hash. Now, why is this important if the important parts of the transaction (the part that deals with the coins) are the same? Its important because of the way transactions work. They always refer to an old transaction. They create a long chain of transactions back to the block where the coins have been mined and created. And this reference works with the TX ID. You create such a chain and the TX you use along the way must not be confirmed. You can create a TX A that sends coins to your friend, who uses them without a confirmation (because he trusts you) to create another TX B to send some coins to his girl. Now this attacker comes in and changes your TX ID. Now there is TX A, the modified twin of A, lets call it A2 and B. B is only valid if A gets confirmed. As it happens durring this attack A2 gets confirmed and B now points to a transaction (A) that is no longer valid. A tried to use the same coins as A2 and A2 is confirmed, thus A will be removed from the memory of all nodes that know about the block that confirmed A2. Your friends girl will be pissed, but its not your fault nor your friends.

In your case I suspect that blockchain.info warned you about a TX like A, but A2 was confirmed so you got the coins anyway. They are the same coins your tradepartner send you, they just arrived under a different TX ID which caused the wallet to get confused.

Many thanks shorena for this post