Now IFF right after you swipe your wallet, and walk out before you have any confirmations at all, someone picks up the wallet and trys to spend it, the following happens.
OTHER:
1) someone imports the private key from the paper wallet they saw you discard.
2) someone creates a transaction to spend the outputs associated with that key.
3) someone signs thier transaction and sends it to whatever node / nodes they are connected to.
4) If the nodes they are connected to have already seen YOUR transaction, then they simply ignore this new transaction.
5) if any nodes they are connected to have not yet seen YOUR transaction, then they send this new transaction to every node they are connected to
6) if those nodes have already seen YOUR transaction, then they simply ignore this new transaction.
7) if there are enough nodes in the system that have not seen YOUR transaction, then eventually a miner sees their tx and scoops it up into whatever block it is mining.
8 ) various miners around the world may do this if your transaction did not relay across the network quickly enough till a miner who finds the magic nonce does so with a block containing one of the two transactions.
9) this miner then submits his nonce (with someones tx) and, if the block contained the the other transaction, you now have been robbed.
Whoever gets to #9 first gets the money.
As you can see, as long as your transaction includes a large enough fee to be properly relayed, and enough seconds have passed for most of the network to have seen your transaction, there is a risk before confirmation but it is very low. If you want to avoid that risk completely, simply wait for a confirmation or two.
If you are really paranoid about it, and the address is somehow associated with your identity (such as being used publicly on a discussion forum), then just be all James Bond and burn the paper wallet after you spend
There. I fixed that for you.