So, the first Mirror Image bug is pretty straightforward - if your opponent’s hand is full, Mirror Image will not create a copy of a creature they control when it comes into play. (Implementation seems to be, if targeting an opponent’s creature, create a card that’s a copy of that creature in their hand, then move it to the player’s hand; if the enemy’s hand is full this fails.)
This I think explains the erratic behavior that some folks have noticed before.
The second bug I discovered in the process of replicating and confirming the first bug, and…well…it’s REALLY weird.
First I cast a Mirror Image when they had a full hand and got nothing, as before. Then they dropped a Gisela, the Broken Blade on the field and ended their turn with 5 cards in their hand. I cast another Mirror Image, targeting Gisela, and got a copy added to my hand - but the card the game actually put in my hand was Brisela, Voice of Nightmares (the melded form of Gisela). Thanks to an extra swap, I managed to drop a third Mirror Image and target their Gisela again, then their turn came around and their Gisela smashed into my Brisela, and when my turn came around I took screenshots.
So…some kind of weird interaction between Meld and Mirror Image’s copying mechanic, I guess?