I apologize for posting this in two places, but I wan't getting much response on the Newbies board (
https://bitcointalk.org/index.php?topic=389719.0) and figured this may be a more appropriate place to ask...
I'm trying to better understand the potential for contracts as described here:
https://en.bitcoin.it/wiki/Contracts. I have a fairly basic understanding of the blockchain and the algorithms operating under the hood, and I've read the original Satoshi paper on Bitcoin.
The possibility for creating low-trust contracts is really interesting to me, but I'm having a hard time understanding the process described in a few of the examples on the wiki.
The question:
What prevents someone from spending the coins included in a partially-fulfilled contract (such as the inheritance contract from example #4) before it's broadcast and verified on the network?
To quote the wiki (example 4 - using external state):
[The old man] creates a transaction with a lock time of the grandson's 18th birthday that pays the coins to another key owned by the grandson, signs it, and gives it to him - but does not broadcast it. This takes care of the 18th birthday condition. If the date passes, the grandson broadcasts the transaction and claims the coins.
If I'm the grandson, what guarantee do I have that by the time my 18th birthday rolls around, those coins will not have already been spent elsewhere? The same question applies to the "pledged" transactions sent to the entrepreneur in example 3 (before the full amount is received and the final transaction is broadcast).
Am I missing something here?