Bugged Interaction Between Ominous Lockbox and Hide in Plain Sight
An opponent played Ominous Lockbox against me (and apparently chose '4'). When I cast Hide in Plain Sight, the Lockbox triggered and copied my Hide in Plain Sight. Once their copy resolved, if I moused over their face-down creatures it showed me a face-up card, instead of just saying "Face down permanent #." When they flipped the card face up, it did match what I had been shown, so I think it's actually bugged, versus just showing me a random card.
When a different opponent just case Hide in Plain Sight against me, I correctly was unable to see what the face-down cards were, so I think the bug was caused by Ominous Lockbox copying my Hide in Plain Sight and the game therefore thinking they were my creatures or that I should somehow be able to look at them.