After sending/flooding transactions, how do we know that a specific transaction is on all nodes?
From a novice, what it means that if someone who isn't bitcoin savvy accepted bitcoin on the deprecated block and gave that person something of value, that person is out of luck, you just lost that item of value.
Previous answers eluded to it, but, simpletons like me look at &quot;So what does this really mean&quot;.  It means, when you get paid in BitCoins, you are not &quot;PAID&quot; until that transaction appears in at least 6 blocks to that chain, since that chain could be voted invalid anytime before that 6th block is reached (It can still be called invalid after that but highly unlikely).
This means, instantaneous transactions using BitCoin are &quot;RISK&quot;.  If I accept bitcoin at my restaurant, it is possible that the chain in which I got paid could be voted invalid, thus, I never received actual payment.  I think the time is currently 10 minutes to &quot;confirm&quot; a transaction is on the valid chain &quot;6 blocks deep&quot;.
A person could gamble and spend money twice, the money doesn't actually get spent twice, but a novice person could be taken advantage if they don't understand how things work.  In short, don't give up anything of value until the chain gets at least 6 blocks deep, then your &quot;payment&quot; is most likely confirmed.