Creature entered the battlefield as a copy (visually) of what I was hovering at the time
On mobile.
My opponent had Glissa, Sunslayer on the battlefield. I had cast a Duress, so I knew they had a Deep-Cavern Bat as well as a second copy of Glissa, Sunslayer in their hand.
They played the bat while I was hovering the Glissa on the battlefield to re-read it's effect. I could tell because the bat disappeared from their hand. On the stack however, it showed a Glissa, Sunslayer, instead of the Bat. The second copy of Glissa disappeared from my opponents hand at the same time as they put the bugged Bat / Glissa on the stack.
As the bugged card resolved, they ended up with 2 legendary Glissas on the battlefield.
I thought the issue was purely visual, but when I tried to remove what I assumed to be the bugged Bat during my turn with a Bitter Triumph (to remove their only flying blocker and win), I was unable to target it.
It alsmost cost me the game, but I managed to quit, restart, rejoin, target the Bat with my removal, and win.