On December 23rd, the OJ community was graced with this new hyper, among other additions:

This card, as written, attempts to pull two cards from the discard pile, producing copies that can be played at any level for free.
...thing is, this is not the discard pile.

This is a list of the most recently played cards and/or activated traps.

The discard pile is an invisible pile containing only the cards which should be reshuffled into the deck, should it run dry. This includes cards that are played as well as those discarded, either by traps or by having a full hand.

This does have moderate overlap with the played card list you can see in-game, but that list doesn't show discarded cards. And unless Sealed or Melting Memories is in effect, you won't have much of an idea what your opponents are discarding.

Oh, and this doesn't fit in any of the following categories:
Cards with variable star cost, like x16 Big Rocket, will NOT become free to play. This is not a bug.

Most cards played or discarded can be copied. Notably:

• Played traps can be copied, but only once stepped on, as that is when they enter the discard pile.
  
• Present Thief, however, must be stepped on twice if the first player to step on it was holding cards.
  
• Hypers can be copied, with a few exceptions.
  
• Leap Through Space (Marking) can only be directly copied when discarded.
  
• White Christsmasher, True White Christsmasher, Angel Hand, Devil Hand, Miracle Red Bean Ice Cream, Magical Revenge, Leap Through Space (Marking), and Leap Through Space may switch forms when copied, if their conditions call for it.
  
• In Santa Scramble, Grown-up Snowball Fight and Snowball Reflector can be copied. However, this has yet to occur in practice.

Cards created from other cards, or otherwise are not part of the deck, cannot be copied. Some other restrictions apply:

• Reproduction of Records cannot copy itself. For balance reasons. Don't ask.
  
• The battle card spawned by Extension cannot be copied.
  
• Red & Blue cannot be copied.
  
• Leap Through Space (Marking) cannot be copied. It becomes Leap Through Space without going to the discard pile.
  
• Evil Spy Work - Execution cannot be copied once activated or discarded.
  
• Dance, Long-Eared Beasts!, Invisible Bomb, Flying Pirate, and Rbit Hobby Shop cannot be copied once activated.
  
• Traps placed by the Minelayer field event cannot be copied once activated.
  
• Cards created by Miracle Walker, Observer of Eternity, and Reproduction of Records cannot be copied.
  
• Cards recalled by Scrambled Eve are placed in the deck, not the discard pile. Thus, they cannot be copied yet.
  
• The card stolen by Tragedy in the Dead of Night is placed in the trap owner's hand, not the discard pile. Thus, it cannot be copied.
  
• Cards stolen by Present Thief are stored in their own list, so they cannot be copied.
  
• In co-op, Sweet Guardian creates two pudding cards, which cannot be copied.
  
• In co-op, all boss cards cannot be copied. They're not healthy anyway.

The following cards discard other cards when played:

• Bad Pudding, which only discards one card.
  
• Miracle Walker and Branch Expansion Strategy, despite their card text stating they are transformed.
  
• Princess's Privilege
  
• Flamethrower
  
• Brutal Prank, which only discards hypers.

Bad Pudding discards itself first, followed by the card it chose to take with it.
The next four discard themselves first, followed by the entire hand they are targeting from left-to-right.
Brutal Prank follows the same pattern, but may not necessarily discard the entire hand.

Note that the small versions of hands next to Player 2 and Player 4's portraits are in reverse order.

For example, if a player steps on a Flamethrower and discards Dash, Mimic, and Invasion (assuming none of them are copies), the two cards at the end of the discard pile will be Invasion and Mimic.

• PhleBuster, for revealing the existence of a separate discard pile
  
• Setsuna, for initiating the deep dive on this entire subject, and hosting an R&D session
  
• Alchius and FancyFishie, for participating in said R&D session