Also, let’s say a human (or Stockfish) has forced mate in 153 moves, but doesn’t see it and instead plays a different move.
Was that an error?
The way we've been talking, if it spoils the win, then yes it's an error
If it's still a forced mate, but it takes more moves, then that's a bit gray
If it's a draw now, but in practical play it's almost certainly still a win, then even more gray
If it's a loss now, and the opponent has a forced mate, but it's almost certainly still a win in practical play...
etc etc.
I'm open to using whatever definition you want.
never play 4.e5 on 3.Nf3
After 3.Nf3 Nf6 the most common move in my database is 4.e5 which has been played by many different GMs.
It may not be the objectively best move, but KG players aren't too concerned with this. They typically want an objectively borderline lost position, with a big white initiative and chances for both sides.
I am using the 2. ...Nf6 sequence as an attempt to refute that line of the King's Gambit. The current analysis of this line appears to be not very good. [i know the response is often 3. e5 by some strong players.]
But if white plays like this there should be no refutation I think?