I got a serious problem with my wallet.
I've always used the default fee which means I let the client decide and set the fee to 0.0000 BTC in the settings.
Sometimes I had to pay a fee, sometimes not. The transactions always went through.
So today I made some transactions but they didn't get confirmed.
I backed up my wallet and started bitcoin QT with the -salvagewallet command.
So those pending transactions got deleted in my client and my balance was correct again.
Then I made a new transaction and included a fee of 0.0005 BTC.
This transactions is still pending and on blockchain it says "possibly never (double spend)" and the queue keeps jumping between rank 1 and 30.
https://blockchain.info/tx/03a9296b9ddb8496f4034e6bf7d55e5d9b80bc6e7548b559550b470c68d4d09a