Lucky Penny

A "Sweet, free money!" button prompt which appears on the last (scroll-able) page of the Suggested Content tab in the Adventure Guide (default hotkey Shift+J) pops up the quest A Piece of Copper that awards a single copper coin and this achievement.

It's not immediately clear to me at the moment how the follow up achievement Curious Coin is acquired, if it's even possible to do so as of the current beta build.

Appears to be part of a small chain of secrets/easter eggs. Will update this comment as more information becomes available.

Edit Nov. 16th 2022: Subsequent quests in the chain appear to be on a weekly lockout. Current progression is Copper -> Silver -> Gold -> Bag of Coins (30g) -> ?

Edit Nov. 22nd 2022: The 5th week prompts the quest "A Curious Coin" with the text:

A single, strange coin sits in your bag, along with a small note: "Thank you for the opportunity to observe you. The information I've gathered has been illuminating, to say the least. This will be my final gift... for now."

The reward is a [item=202107] worth 100g and the Curious Coin achievement. It very much seems that we'll have a follow up of some sort to this sooner than later as Dragonflight progresses. In the meantime, I'm sure lore enthusiasts can speculate on who the mysterious benefactor might be. New character, perhaps?

somebody at Blizz probably thought to themselves "I wonder if anyone ever looks in this thing?" and came up with this idea

1) Open Adventure Guide (shift-j or the skull on your micro menu)
2) Click on the second tab on the bottom called "Suggested Content"
3) Left pane of that frame has arrows in lower corners - hit right arrow
4) Continue hitting right arrow until the end
5) First time, last panel reads "Copper Coin," click "Sweet Free Money!"
6) Complete the quest
7) Each week the next entry in the series will appear in the last panel
8) Repeat until questline is complete (ends at week 5's "Curious Coin")