Si tu as vu la carte du dessous de la bibliothèque, cette dernière n'est plus considérée comme étant mélangée, donc la bibliothèque doit être remélangée.
De plus, en REL compétitif ou professionnel, tu est obligé de remélanger le deck de ton adversaire après qu'il te l'a présenté.
Voici ce que disent les Tournament Rules:
If a player has had the opportunity to see any of the card faces of the deck being shuffled, the deck is no longer considered randomized and must be randomized again.
At Competitive and Professional Rules Enforcement Level tournaments, players are required to shuffle their opponents’ decks after their owners have shuffled them. The Head Judge can require this at Regular Rules Enforcement Level tournaments as well.