In a recent game I thoughtseized my opponent, revealing a fable of the mirror breaker and two copies of Sarkhan, Dragon Ascendant along with three lands. I discarded the fable, leaving the two Sarkhans visible in their hand. On their next turn, they play a land, and then cast one of the Sarkhans. When they did, the other copy was no longer visible in hand, and the one on the battlefield was bugged. I couldn’t target it with removal or block it with my own creatures. It could still attack me, but I was unable to interact with it in any way. My opponent had nothing in play that should have prevented me from targeting it or blocking it. Not sure what caused this and I have never seen anything similar happen, it was bizarre.