I'm aware the PTR economy is already quite broken but this is still an amusing bug.
I'm not entirely sure how to replicate it but it seems fairly simple from my PTR experience.
1. Copy over a character from the Ulduar era with outdated Conquest Badges (this requires the character being completely inactive/unplayed for several years). This means even after Blizzard mailed you gold you did not login to pick it up.
2. Attempt to login, your character will show up on the server but your game will lock up at the loading screen.
3. Use a paid character re-customization (not changing factions, just race changing on your own side).
3. The Emblem of Conquest exchange rate is bugged and instead of being mailed the appropriate amount you'll receive letters with the sum of 214,748 gold, 36 silver, and 47 copper each. A self replenishing and unlimited source of gold for at least 30 days (afterwhich I'm assuming the letters expire). In minutes I literally handed out 20 million gold on the PTR and the envelopes still kept coming. It was easily repeatable with other copies of my character.
The bug stems from a change implemented in 4.01
"Anything below Emblems of Triumph, including badges from The Burning Crusade dungeon and raid content, have been converted into gold and sent to players via the in-game mail system. "
I've copied my priest over before and never seen anything like this, something obviously broke in their latest patch.