transactions – Is it theoretically possible to have an inverted nLockTime?


This is intentionally not possible.

If a transaction would be valid in block X, we want it (absent double spend) to remain valid in any successor of X. This guarantees that (temporary, otherwise harmless) forks that result in small reorgs will never invalidate unrelated transactions – which will just be able to ve mined again in the new branch.

It also simplifies wallet assessment of incoming transactions. Imagine a transaction that was just confirmed in the last block where it would be valid. 1-block reorgs are infrequent, but not rare. You’d need special logic to detect this and factor the risk of a reorg that permanently invalidates the transaction into the decision of when to accept it. It’s even worse for unconfirmed transactions, though there are other issues with those as well.