as an example, I have the wallet with address-A (with 10 BTC)
If I send 1 BTC to someone (address-B) then in transaction I see
address-A > 1 BTC > address-B
address-A > 1 BTC > address-C
after, on address-A I have 8 BTC, but my bitcoin-qt show me 9 BTC (as it must be)
why it makes sends to strange address-C ?
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Found an answer here https://en.bitcoin.it/wiki/Change
When you send money to another address, the client selects one or more unspent outputs to get a sufficient amount to cover the amount you are sending. Any excess is returned to your wallet using a change address.
In your case, it looks like you had an unspent output of 2 BTC. Since a transaction completely consumes its inputs, the transaction needs to send 1 BTC to your specified address and 1 BTC back to your wallet to account for the 2 BTC that were consumed.
Changes addresses are typically not displayed by the client. If you are running Bitcoin-Qt, you can check all of the addresses in your wallet by entering the command
listaddressgroupings
in the debug console.
