Suppose I spend funds from an address, but then there's a software bug, or user error, or somebody sends IOTAs to that address afterward, and now I have a used address with a non-zero balance!
What can I do to secure those funds? Would sending the funds to a new address do anything? Or, should I consider those IOTAs to be lost?
You should send as soon as possible all the IOTA from that specific address (say ADDRESS9X) to another address of yours from which you never sent (say ADDRESS9Y). If this transaction succeeds in being confirmed by the majority of the network (or by the COO as it is now) you are OK.
But how do you do this with the current wallet software?
Let's assume you are using the current wallet software to spend your IOTAs. Since you have spent IOTAs from ADDRESS9X (which is generated from your seed at a certain index, say index1) it means that you should have no IOTAs at any address generated from the same seed at a lower index.
Now, someone sends n IOTAs to ADDRESS9X.
So, if you know that ADDRESS9Y is generated from that same seed at a greater index (say index2) and that you never used it to send IOTAs, then you should be safe to use the wallet software, login with your seed and send that precise amount (n) of IOTAs to ADDRESS9Y.
The Iota are on a seed which was used already. You can send them from there, but you will reveal a big part of the addresses private key. So sending everything at once should be the way to go. But it is important to get the transaction confirmed fast. because as long as it is not confirmed someone could still steal them. In general, you should imidiately call the one, who sent the money to this address and give him a new destination. So your money stays safe.
