Again:
If any of these situations happen--you balanced the adventure wrong by choosing the wrong encounters. Or you are using the wrong ruleset.
That's inherently that the adventure or GM screwed up, but sometimes only obvious to the GM at run-time.
As for "wrong ruleset", I would recommend not going down that discussion path as it can be viewed as hostile. My peeps play D&D. period. There are no other rulesets for consideration. therefore, all problems have to be resolved within the D&D framework. Suggesting other rulesets is akin to not listening to the customer. And thus, is not conducive.
The players presumably showed up to play a game where the monsters have an x chance of hitting them and (once they hit) have a y chance of doing z damage. Any outcome in that range should be acceptable to them or they should be playing a different game.
I doubt the majority of players give much thought to the exact statistics that ruleset A offers versus ruleset B. they show up to play what their friend is willing to GM. or avoid that game because they can't stand the rules. Not much actual math involved.
The GM then planned an adventure with the possibility of a certain number of foes being encountered over a certain amount of time. If any possible outcome within that range is not acceptable--they should have written the adventure differently.
Again, this is inherently true. Fudging, to a GM who's made this mistake is how they silently resolve their mistake.
"Bad luck" is part of the game and the set up. If players are willingly playing D&D it's because they have accepted the possibility of "bad luck" (regardless of their mood). If you want to insulate players from bad luck, do not play a game where bad luck has that much power or do not set up the game such that that many encounters are possible in that amount of time.
Once again, telling me not to play D&D just ticks me off.
We've been quite happy playing D&D for 20+ years with whatever fudging is or isn't happening without some outsider who isn't even in our group telling us to play a different game.
Players dying because they misjudged an encounter is part of the game. Judging the encounter properly is part of the skill involved in the game.
And for some GMs who fudge, that PC would be dead. The fallacy is that a fudging GM is always fudging to protect the player. I posit that the first reason a GM fudges is to protect the player from the GM's mistake.
given that the adventure only says the monster does 1d8 damage because the GM wrote that last night, and the GM effectively changed his mind tonight at the game is ludicrous to assume that it is a sin to change that.
If the GM is doing everything right (giving signals, opportunities for clues, etc) and PCs die because they assumed they were invincible--they're losing because they played D&D poorly. Like in any other game: play poorly and you
lose.
If you don't want the possibility of death in nearly any encounter--play a game that isn't D&D. There are lots of games out there that allow players only to die when they want to or when they're "ready".
IF you signed up to play, it means you're ok with 3 kobolds in a row rolling 20s and getting criticals on the same PC. That's baked into the combat system of this particular game.
And in this scenario of a party choosing poorly, a GM who fudges lightly is likely to let the PCs die. their mistake is not his mistake.
I would NOT agree that signing up to play means tacit acceptance or even cognizance that 3 crits in a row could happen on the same PC. People are very poor at seeing all possibilities and acknowledging that they could happen.
People sign up to play, to have a good time with their friends. They chose D&D because it's fun most of the time. they don't tend to think deeper into contract law on what terms they accepted when they sat down to play.