Was purchasing some Steam games for Bitcoin and this error message popped up:
"Error: The transaction was rejected. This might happen if some of the coins in your wallet were already spent, such as if you used a copy of wallet.dat and coins were spent in the copy but not marked as spent here."
Happened in this order:
1. Sent 2 BTC which went through fine and showed up on the blockchain.
2. Went to send the remaining 4 BTC owed and then I got some sort of runtime error and had to restart my computer. Fairly certain that the error occurred due to low disk drive space. (A transaction with 0/unconfirmed status shows -4.00BTC from my wallet although nothing shows up in the blockchain to the address I sent it to)
3. Waited a bit for confirmations to show up on the first 2 BTC (2-3 confirmations now) and nothing about the 2nd transaction appeared on the blockchain so I went to re-send the 4 BTC and got the above error message. My wallet now shows a 2nd -4.00 BTC transaction that didn't show up on the blockchain either although my wallet balance is showing 4BTC less as well.
Google'ing the error message has some results but no actual explanation of why or how to fix it.
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Edit: Apparently one of the 2x 4BTC transactions have shown up on the other person's wallet, although the blockchain shows both as unconfirmed.
Here's the blockchain page:
http://blockchain.info/address/1DTVTpqXufEb1CwfbSMNNvNJRs1yLciMhd