Quote Originally Posted by Craft (Cheese) View Post
"You see strange glowing symbols on the wall."

"Okay, I roll Knowledge (Arcana) to see if I know anything about them."

The problem with this is the DM has probably already decided what they want you to know about the strange glowing symbols and what they don't. And if there's something that actually matters in it like a critical plot clue or something, they're not gonna leave that up to the roll of the dice.

"Whoops! Looks like you failed your Heraldic Lore check, so those guys you attacked were actually your allies. Now both the incumbents AND the rebels want you dead! Have fun!"
If there's a reason that you should know something, then you know it, no roll required. The knowledge skills, like all skills, are only supposed to come in when there's some sort of uncertainty. If you're a farmer, you probably know a lot about cows. If you grew up in the desert, you probably don't know anything about fish. This is exactly the kind of case where you would roll with advantage or disadvantage, at the DM's discretion. The DM also has the option of simply having you auto-succeed or auto-fail, if there's no possible way you would know or not know something.

There's no need for special rules in the knowledge skills to cover these situations, because they're covered in the rules for all skills. It explicitly says that if you're just walking down the street, you don't need to roll balance to not fall over. Similarly, if a clumsy (8 dex) wizard tries to walk across a thin, rotted rope strung between two ships being tossed in stormy seas, you don't roll for that, the DM just tells you how badly you fail. The same rule applies when your dull (8 int) rogue tries to recall the jewelry being worn by the Djinni involved in the ancient conspiracy between the Elemental Plane of Fire and the Elemental Plane of Slightly-Less-Fire that occurred three thousand years ago.