Is it a bug? A plane?! Nope! It's a game mechanic. If you can see a specific card revealed in your opponent's hand, there's likely a very good reason for it.
We don't expect you to memorize every card that reveals itself due to the myriad of spells, keywords, and effects that can appear on the board. Should your opponent's card be revealed to you, it will remain visible even if it remains in your opponent's hand.
Let's take a look at Katarina. Her kit revolves around attacking and Recalling to the befuddlement of her enemies (you can learn all about Katarina's level-up conditions here). When Katarina's Recalled, you already know she's there, sharpening her knives and coming for you. That's why you can see her revealed in the top left corner. You can always hover over the revealed card to see all the details on hurt coming your way.
