But at which point does this transaction become truly invalid?
1/4 of nodes
1/2 of nodes
3/4 of nodes
When is it clear that a transaction is invalid? Is it really a number of nodes or is it the consequences you're faced with when certain nodes do not recognized your transaction?
I Don't think what I'm talking about as to do with a point of failure. I don't see recognizing a transaction as valid or not to be a failure or not of Bitcoin.