Can someone explain double spending to me?

Question

I am restoring a backup of my Bitcoin Core wallet on a new PC. I downloaded the complete bootstrap.dat and placed it in ~/.bitcoin. I thought using the bootstrap would make things faster, but importing from disk seems just as slow. Oh well.

Anyway, currently Bitcoin Core has imported blocks through Jan 2014. I made bitcoin transactions after Jan 2014 that aren't reflected in my wallet. That is to say, I have a higher balance available in my wallet than I really should.

Could I double spend these coins by sending them to another bitcoin address that I own?

Answer 1

No you cannot. When you try to send a transaction to the chain which is not allowed by the chain (so sending more coins then you own) results in a invalid transaction. The case now is that your program didn't process your recent transactions, and therefor thinks that there are more bitcoin in your wallet then there really are. Otherwise it would be far to easy to doublespend: just reinstall Bitcoin Core and have a wallet which held a lot of BTC prior a specific date. If you are interested in reading more about doublespending (which I think you are not, just wanted to know if you were possible right now) you can read this.

Answer 2

No, it's impossible.

You first need to realize that the Bitcoin network is so successful because it is designed to be resilient from attackers cooking their transactions and Bitcoin protocol messages. It is not some closed-source proprietary crap that only relies on a consistency of some nonfree software installed on your computer and breaks down as soon as somebody finds a way around it.

In other words, the Network does not care what your local Bitcoin Core thinks or assumes to be true. Even if you managed to trick your client to send out the transaction, you would only be fooling yourself as no one else would believe it (because they all know it has already been spent before).