Bug: Jace did not mill the opponent out completely and cost me a win even though he should have milled them completely
Opponent had less than 45 cards in their deck. I had a Chandra, Hope's Beacon on the battlefield and cast Jace, the Perfected Mind. Then I ulted Jace targeting opponent to be milled for X=5 x 3 = 15 cards. Then I cast Return the Favor, copying the Jace ult on the stack. Chandra copied the Return the Favor, and with the copy of it, I copied the Jace ult again.
The result should have been the opponent getting milled for 15 cards three times, a total of 45 cards milled. However the final Jace ult instead of milling the opponent for 15 cards, it only milled them for TWO cards instead of the rest of the cards in their deck (they had four left at this point). At the end of the game, they only had 43 cards in their graveyard instead of 45 like they should have had.
As a result of this bug, I lost a ranked match. Please credit me for the win on this one. It's ridiculous for the game to have such bugs at this point.
Here is a video of it happening: https://www.twitch.tv/videos/2416541105
I'm still laughing at the end because I didn't realize that it didn't completely mill out the opponent like it should have...

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Muggle_of_the_Coast#19144 commented
Opponent had (at least one) Progenitus in the deck. That card reads "If Progenitus would be put into a graveyard from anywhere, reveal Progenitus and shuffle it into its owner's library instead."
This includes milling. So every time Progenitus got milled, it got unmilled, and after all your Jace triggers and copies the Progenitus/es was/were still in their deck.