When someone gets a status of "0/unconfirmed" on a transaction for a long time - what could he be doing wrong? What should he check to solve this?
bitcoind from the command line with the
-rescan option. It may be that your transaction was erroneous or a double-spend attempt and it only exists in your client. You could double-check the receiving address in blockexplorer to see if the transaction ever came through from the rest of the network's perspective. If this is a zero transaction fee transaction (or is just otherwise unlucky) it might still be in-queue to be included in a block. Bitcoin Charts has a list of in-queue transactions you can check as well.
Assuming blockexplorer sees the transaction, your client probably isn't downloading blocks and may not even be connected at all. Check that the block number in your client matches the block number at blockexplorer (or any of a number of other sites), check if your client has 0 connections. If you have no connections, try the troubleshooting steps from this question. Worst case scenario delete everything from the Bitcoin data folder except for wallet.dat and re-launch the client. Several hours later when you've finished downloading a fresh copy of the block chain, your 0/unconfirmed transaction will probably be resolved.
One common cause of this condition is manually forcing a zero transaction fee. If you force a zero transaction fee on a 'large' transaction (one that gathers Bitcoins from many other transactions or sends Bitcoins to many different sources) other nodes in the Bitcoin network will refuse to relay your transaction.
While there are some nodes that will relay all valid transactions, your client will only send the transaction out about once every half an hour or so. As a result, it can be a long time before, by sheer luck, your client happens to send the transaction to a node that's willing to relay it (that happens to be able to relay it to other nodes that are similarly willing).
When users are quick to close the client, the transaction being sent will not get re-broadcast if for some reason it didn't propagate sufficiently when the first attempt was made.
So before even trying anything -- leave the client up and running on a reliable connection for maybe an hour. If it still stays at 0/unconfirmed, then other troubleshooting is warranted.
You can search for your transaction hash on http://blockchain.info/rejected. It will say if your transaction hash did not meet the minimum fees or my client could not connect it's inputs.
If a transaction has been unconfirmed for three days, clients will delete it. This generally happens when you force a zero transaction fee.
This far the bitcoins are so called send and my btc shows 0.00 in wallet I did several deletes of the whole thing but the btc is not restored or in being send at all I left it open about 4 hours after it downloaded but nothing changes the -3.7737 btc keeps showing on the right side i did 2x bitcoind -rescan before but the download of the chain takes a long time as well even the given stransaction id i could not find in the list i redeleted the files now again but untill now nothing has changed