what happens if a node passes an invalid transaction? When the error becomes evident and who will notice if nobody has reported it?
Blockchains record states. This effectively means that miners provide a transaction history of every single coin up until their genesis. Therefore you can only spend any coins if you have them in the first place (according to the current state of the blockchain). The only way to corrupt this system (in the case of the Proof Of Work algo) is to re-mine previous blocks so to change the history of transactions. You could use this to first make it seem you paid for something to someone then change this transaction by spending the coins to your own wallet and redo the work while out competing the rest of the network. This kind of corruption attack is called a double spend which can only be done if you control 51% of the network for over 20 minutes. Which at present is extremely costly. You also asked about when an error becomes evident, that depends on what kind of error you mean. If by error you mean spending your coins wrongly eg; you spent your coins to a wrong but valid address good luck finding the owner of that address and begging for your coins back or If you spent your coins to an invalid address and your wallet does not check for this bye bye coins. If by error you mean double spending, this means that there are two non-corresponding states of the blockchain. Which causes the blockchain to split into two different chains each reflecting their truth. Only whilst you outcompete the rest of the network for long enough you can successfully pull off a double spend. But notice that the system is fully decentralized thus "reporting" is done by running computers doing math and contributing to the state of the network not by spamming a support staffer for help or anything like that. Hope this helps! Good luck on your learning journey!
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