I started the standard client with
--rescan instead of
-rescan after switching wallets and before synchronizing with the latest blockchain, and made an going transaction.
This transaction got "stuck", never validating, even after the client caught up with the chain, after multiple restarts (last one with -rescan). I now have a too high balance shown in the client, and the tx never went out (I checked it in Blockchain.info).
After the blockchain was synchronized, I create another similar tx (same amount and target address), and that one went through ... but the bad tx is still shown in my client.
What should I do about it?