In this episode, Jace makes a reference to demon pox. This relates to the prequel series, The Infernal Devices, in which Will Herondale is constantly referencing demon pox.
Clary apologized to Jace for the other 10% chance that her plan could fail because she was only 90% sure, and slapped him shortly afterwards as part of her plan; this is in reference to the books and from a deleted scene from The Mortal Cup where Jace was 90% sure that she was a Shadowhunter when they first met and that drawing a rune on her wouldn't kill her, for which Clary slapped him for the 10% chance that she could have died.
The episode title references the Tarot cards in which the Mortal Cup is hidden. However, the Ace of Cups is not one of the 22 cards of the Major Arcana; it is part of the minor cards.
The episode shares a title with the name of one of the episode writer's favorite Speedy Ortiz albums.