The players have no right to any of the DM's information.
This is not a statement of necessary truth. It's an expression of a playstyle preference, and I would say a fairly narrow one at that.
For instance, if I'm GMing a classic D&D module, and the treasure key says that there is a magic sword in the chest, and a player whose PC is standing next to the chest casts Detect Magic, then the player absolutely has a right to the information that the spell registers the presence of magic in the chest.
That's the whole point of those detection spells and abilities that are so common in classic D&D - that when players use them in relevant circumstances, the GM is obliged to hand over GM information, in the form of true answers about what is written in the dungeon key.
I really see no reason why a DM should feel guilty, or a need to explain, making small tweaks in the design of his/her adventure that ultimately benefit players.
This is a different process from fudging dice rolls - it is changing the fiction, as opposed to interfering in the mechanics of action resolution.
For some players the two things - changing fiction, fudging dice - may be equally permissible or impermissible. But there will be other players for whom one is permissible but the other not.
Fudging a die roll is a tool at the DM's disposal. Like everything at a DM's disposal it is not inherently good or bad. How it is applied is what matters.
I'm not sure what this is meant to mean. Applying fudging at my table is bad. But that's not really about "how it is applied", is it?
Fudging is acceptable to some playstyles but not others. As a generalisation, the more comfortable a group is with the idea that the point of the game is for the players to experience the GM's world/adventure/story, the more comfortable the group will be with knowing that fudging is taking place.
For my part, I incline towards
Luke Crane's view, talking about Moldvay Basic:
I've a deeper understanding why fudging dice is the worst rule ever proposed. The rules indicate fudging with a wink and a nudge, "Don't let a bad die roll ruin a good game." Seems like good advice, but to them I say, "Don't put bad die rolls in your game."
To expand on the point: The players' sense of accomplishment is enormous. They went through hell and death to survive long enough to level. They have their own stories about how certain scenarios played out. They developed their own clever strategems to solve the puzzles and defeat the opposition. If I fudge a die, I take that all away. Every bit of it. Suddenly, the game becomes my story about what I want to happen. The players, rather than being smart and determined and lucky, are pandering to my sense of drama—to what I think the story should be.
So this wink and nudge that encourages GMs to fudge is the greatest flaw of the text.
If the player's can't if tell the DM is fudging, it doesn't matter if he does or not.
If the players can tell the DM is fudging, he's shouldn't be doing it.
If the players suspect a DM is fudging, something has gone wrong.
Like [MENTION=6790260]EzekielRaiden[/MENTION] upthread, I think there is a tension here. If fudging is OK only when the players neither know nor suspect it, then there is something fishy about it. RPGing is a social activity, so why can't the methods all be public to the participants?
Fudging the die rolls so that the baddies always hit, make their saves, crit or generally make it feel like the DM is being vindictive is a bad thing. Does that help?
