Kids definitely absorb those rulebooks in a way adults don't, that part is spot on. But the sad part isn't really that a 12 year old knows more, it's that nobody at that table ever bothered to double check the guy running it. Four years of bad calls usually means the group just trusted the teacher because he was the adult in the room, not because they couldn't have caught it themselves. Half the folks at an FLGS have the PHB memorized too, they just don't want to be the one arguing with the DM mid session. So it's less about the kid being impressive and more about a table that never questioned anything.

Grinder change is the big one people sleep on. Had the same thing happen with a Guatemala roast that I let go a bit long. Thought I torched it. Two days rest and it was all cocoa and brown sugar. The old blade grinder I had was crushing the beans uneven and making everything taste like ash. Went to a burr grinder and it was night and day. Dark roast gets a bad name because most folks never had it ground right.