Thank you for the clarification, so basically they don't included the parent transaction in the same block, and the parent was still unconfirmed.
No. They did include both the parent and child transaction in the same block. It's just that they included the child before the parent in transaction data. So, when the nodes would verify the child, they'd deem it invalid as it'd spend missing UTXOs. That is called an orphaned transaction.
This is the particular part from validation.cpp that prompted the error:
https://github.com/bitcoin/bitcoin/blob/c9f288244b8d183e09a917025922b99e3368ef78/src/validation.cpp#L777C1-L796C6So was the developer tested this deliberately or was it an accidental mistake.
That's a mystery for the time being.
So could the developer's mistake/error/system test be duplicated a lot at a low cost?
Absolutely no clue. We'll wait and see. I'm afraid this didn't happen deliberately.