In the video (final Coinbase section), you mention that only Anders can transact to others in Block 2, because only Anders has a historical balance to transact from.

It would be a great enhancement if this final Coinbase implementation also made blocks Red/invalid if there are any attempts to transact from a source with insufficient balance (or maybe a different color-code, like purple? to indicate a different kind of invalidity than the Red for incorrect hash chaining)

(Another thought: maybe the "Mine" button is deactivated until the transactions are valid according to historical balances)