[MENTION=23751]Maxperson[/MENTION] where does it state in the 5e rules that the DM can change or ignore die results after the fact?
Your question is irrelevant. A player is not the DM. The roles and powers of the player don't allow the player to do many things that the DM can. You might as well ask if it would be wrong in the minds of DMs in favor of making encounters if it would be wrong for the players to decide to make encounters.
Page 235 of the DMG under Running the Game, where it says "Rolling behind the screen lets you fudge the dice if you want to." It doesn't get much clearer than that.
DMG page 235 said:If you roll the dice where the players can see, they know that you're playing impartially and not fudging rolls.
Third bullet point about dice rolling said:... Don't distort die rolls too often though, and don't let on that you're doing it. Otherwise your players might think they don't face any real risks - or worse, that you're playing favourites.
If you've been paying attention to the railroading threads, not all railroading is bad. It depends on what it is, what the purpose is, and player buy-in. Not all fudging is bad, either. You are trying to apply absolute standards to something that is not absolute. That's why your arguments break down.
The rules serve the DM, not the other way around. The rules cannot require anything of him or her. The result of the successful attack roll is not uncertain. You don't need to roll the damage dice in this case. The 1-hp wizard is knocked unconscious after any successful attack roll because those are the failure conditions of the stakes. No fudging is occurring.
A player can physically do the action I'm describing. Heck they might accidentally do so by rolling a d12 instead of a d20 by mistake or just reading the numbers incorrectly due to poor eyesight. The player cannot physically make an encounter appear for the players to fight (though some of those conjuration spells come close when things go badly, I kid, I kid).
So a player (and let's assume they're not new so no excuse on that front) has just done the inconceivable and misstated their roll to be less than it was. Assuming it wasn't a mistake you'd... put it in with any other intentional rules violation regardless of motive? Or something else? I don't hold that there is one right answer here either, others have even stated they'd be fine with it. I'm just curious of your reasoning.
And I imagine it would vary table to table. A table where people rotate GMs campaign to campaign (or even mid campaign) is probably going to have a different feel than one that has a particular person as the GM the majority of the time. Out of curiosity where would your current gaming group fall on that scale?
But there's no practical difference. In one case, you fudge the dice so they deal less damage. In the other case, you roll an attack, hit, look at the PC's HP, see that it is one, and decide that instead of dealing 10 damage the attack will knock the PC unconscious.
Both are changing the rules on the fly as it suits the DM. I do not think that this is wrong, I think it's great. But there's no difference between that and fudging rolls. The effect is exactly the same. Unless the monster stat block says that the attack made by the monster cannot ever kill a character, only knock them unconscious. But then that's a monster design, and the issue would never occur in the first place.
Note, there is more to that part than that single point.
The section you are quoting presents a number of options, not just carte blanche for fudging. I choose the first option, you don't. And that's fine. It's a play style thing.
Even in the DMG advice, the idea of distorting die rolls comes with caveats and limitations - keep it a secret and don't do it too often. For me, and, again, purely for me, any distortion is too often and, the fact that you have to hide it says to me that it's not groovy. You only hide stuff that you know players would object to. If the players would object to something if you did it, then don't do it. AFAIC, it's that simple.
I guess railroading & fudging are natural bedfellows - if you like one you probably like the
other.