This happened twice during my last match - Etali entered the battlefield and exiled 2 cards. I cast my opponents card first and it did not allow me to cast my card.
The first time it exiled Zealous Conscripts for me and Ertai Resurrected for my opponent, cast Ertai first and was unable to cast Zealous
The second time it exiled Snuff Out for my opponent and Through the Breach for me. I didnt have targets for Through the Breach so I cast Snuff Out and sure enough I could not cast Through the Breach. I wanted to cast it still to have it in my graveyard instead of exile, but I also wanted to see if I could replicate the issue.
Match number 264481687, Game number 861183242
In both cases there were minimal non-land cards on the board and nothing that prevented casting, it certainly seemed related to casting my opponents card first.
The first time it exiled Zealous Conscripts for me and Ertai Resurrected for my opponent, cast Ertai first and was unable to cast Zealous
The second time it exiled Snuff Out for my opponent and Through the Breach for me. I didnt have targets for Through the Breach so I cast Snuff Out and sure enough I could not cast Through the Breach. I wanted to cast it still to have it in my graveyard instead of exile, but I also wanted to see if I could replicate the issue.
Match number 264481687, Game number 861183242
In both cases there were minimal non-land cards on the board and nothing that prevented casting, it certainly seemed related to casting my opponents card first.