August 10, 2010 - Stone Man lost forever 9,000 BTC.

Here are the details.

  1. (Stone Man ) Bought 9,000 BTC on one of the exchanges over time.
  2. Transferred them to my client running on a linux live CD distro of Debian.
  3. Backed up the wallet file to a flash drive.
  4. Sent 1 BTC to myself
  5. Closed client before any confirmations
  6. Shut down system (wiped system disk loaded into memory and therefore the ./bitcoin folder
  7. Loaded system back up
  8. Copied old wallet.dat file into ./bitcoin folder
  9. After some confirmations appeared the balance was 1 BTC and there was a transaction saying I spent 8,900 BTC to an address I did not recognize
  10. I read on the forum threads that people have had problems like this but it seemed only when they were trying to double-spend by sending coins to another address and reloading an old wallet file

Source: https://bitcointalk.org/index.php?topic=782.msg8620#msg8620

When you do a transaction you always spend ALL of the coins in that address, part of the money goes to whoever you’re paying, and there rest is returned to you an a new ‘change’ address. (This helps your anonymity as someone watching cannot tell which amount you spent and which amount you kept.)

  • Before transaction:
    • Address1: 9000
    • Address2: 0
  • After transaction:
    • Address1: 0
    • Address2: 1
    • Address3: 8999  <- No backup, private key of this account NOT in his wallet

Today BTC uses a UTXO set, whenever you send a transaction, the unspent remainder, known as the change output, is saved to a separate address. Nowadays, that address automatically send back remaining sum to the sender.