The process of checking whether an input exists is crucial; if the corresponding output (prevout) does not exist, the input script fails validation. This is part of a broader system of checks during the block connection phase, which allows for concurrent verification across blocks without needing to follow a specific order.

Several key checks are outlined to ensure the integrity of transactions. These include ensuring there is no forward reference, meaning a transaction cannot refer to future transactions as inputs. Another critical check is to prevent internal double spending by verifying that the prevout has not been spent in any previously confirmed block. Furthermore, it's essential that the prevout exists and can be executed successfully, indicating that it must be part of the current or a previously confirmed block. Lastly, it is verified that the prevout has not been spent in any previously confirmed block, which reiterates the importance of preventing double spending within the network.

This detailed examination into the transaction validation process highlights the complexity and robustness of security measures in place within blockchain technology to ensure the fidelity and reliability of transaction history.