Generally speaking it's considered bad practice for a GM to call for rolls that literally no one in the party can succeed at, but as with anything in tabletop roleplaying there is nuance. There could be a narrative reason for the player to not know just how difficult something is and you don't want to give it away by just telling the players they can't succeed. If the most capable member of the party rolls a 20 and fails then the "reward" is the narrative of the attempt and learning what you're up against. Or maybe someone in the party *could* succeed but for whatever reason the child-prodigy wizard with a strength of 8 wants to try lifting the portcullis. It wouldn't make any sense for them to actually do it.

This reminds me of it's always sunny when Charlie, who is illiterate, realises he can read Irish gaelic because his childhood penpal, who turns out to be his father, wrote to him in Irish.

I can't believe nobody has mentioned it yet. The runes clearly say "Is this Loss?" It's written plain as day

"Ok your dumbass character gets the genius idea to ask someone who knows more about runes to read them."

Bad faith and pedantry aren't the same. The comic very clearly implies that the nat 20 caused their dumbass character to be able to decipher the runes. If it didn't, the player wouldn't have announced "Nat 20", but the actual score, wirth modifiers taken into account.

it was Frodo who figured it out

Nat 20 is very, very commonly used by GMs to mean "critcal success" in or out of combat, no matter the explict rule. Same goes for nat 1 being a "critical failure." Why? Because it makes the game better for everyone to have these rare rolls rewarded or hilariously punished.

In BG3 (which mostly follows d&d 5e rules) you succeed in a skill check with DC 30 on a natural 20 even if you have less than +10 as a modifier on the roll Are you sure a 20 has no special meaning in checks in d&d (I presume you mean in d&d as it's the most popular system)

Yes he is and no it does not. That is a common house rule Larian implemented into BG3, but it is not part of the original rules of DnD 5e.

I'm glad you know them so well. In what way is such a popularly used rule not a rule?

Why not roll Luck?

I take this more as the character just guesses and somehow gets it right. Or at least close enough.

Critical Skillchecks is an optional rule in the Dungeon Master Guide 5e on page 242. They are as optional as feats and multiclassing.

I wasn't arguing in bad faith. Everything I said was factual, honest, and trustworthy. You are correct that a nat 20 caused them to be really smart and have the best chance to read the runes (nothing shows them actually reading it to be fair). This is because the nat 20 is the highest possible roll available to the player, before modifiers are added! In many instances, rolling that high passes skill checks up to "Hard" (according to the DMG) automatically unless you have some negative modifiers. With the assumption that this player was attempting something actually attainable, this high roll is translated as the character having the absolute epitome of their ability to translate the runes (whether or not it is successful.)

Thanks for being a good sport!

Lindybeige once put dice rolls into a different perspective. Rather than the dice describing how well the action was performed, his suggesting was that the dice would describe the environment. In this case, that would describe how complicated the code is. One of his examples were for athletics, where he thought of it as describing how tall a wall is. Your athletics was 14, this wall turned out to be 15, sorry, you just barely didn't make it over.

The bottom bit looks like Loss; ~~:.|:;~~

"Umm...'mellon'?"

I think you answered your rethorical question yourself: If it is not in the official books, it is not an official rule. And I would not say that they leave it vague. To quote the PHB: "To make an ability check, roll a d20 and add the relevant ability modifier. As with other d20 rolls, apply bonuses and penalties, and compare the total to the De. If the total equals or exceeds the DC, the ability check is a success [...]. Otherwise, it's a failure, which means the character or monster makes no progress toward the objective[...]." That does not leave much room for interpretation. It plainly say that if the exceed, then they succeed and if they don't, than they fail. Yes they don't make an explicit remark about critical results, but they don't need to, because such a rule was never meant to exist in 5e aside attack rolls and death saves. Not to say that you can't make it a rule at your table (same as with everything else), but there is still not much room for missunderstanding the official print.

Also a Nat 20 / Critical Skill check doesn't guarantee success, just the best possible result.