I was reading this thread about a client fork that does not force transaction fees and I noticed the big red warning saying that some coins might get lost if I am not careful. What does this mean? What should I do if my transaction is not accepted?
If your transaction does not get accepted, you have a few options. Right now, at least for Bitcoin itself, there are still enough miners willing to include any valid transaction. So if you just keep waiting, it should eventually get confirmed.
You may also wish to stop your client and then restart it, manually forcing it to connect to a client known to accept transactions without fees. It can help to disable incoming connections to maximize the chances your transaction will go to the client that will accept transactions without fees. It can take several hours for the transaction to get out, even if you do everything right. (The client intentionally 'dribbles' out transactions to try to hide their origin. This completely fails for transactions with too-low fees, but the client still tries.)
Otherwise, you can abort the transaction attempt. Stop your client and then make sure the transaction is not in limbo by checking the list of pending transactions. If it's not there, you can stop attempting the transaction to recover the coins. Ideally, you made a backup of your wallet before you attempted to make the transaction -- reverting the wallet and restarting the Bitcoin client with the
-rescan option will remove the transaction. Otherwise, you'll have to (ouch) edit your wallet file to remove the transaction.
David's answer is outdated by now. Today, transactions without fees generally don't get any confirmations.
A new startup flag for Bitcoin Core has been added since, which removes unconfirmed transactions:
Note, that unconfirmed transactions that were propagated in the network and do get confirmed later will be rediscovered by your wallet if they are confirmed and included in a block.