The Castafiore Emerald: Part 1 - S3-E7
Revealing mistake: When Castafiore is showing everyone the Castafiore emerald, after she says "He named it in my honour", the shot cuts to a close-up of her hand, saying "The Castafiore Emerald." At the start of the shot, she has two rings on two fingers, then she turns her hand and the two rings vanish from her fingers entirely. (00:09:00)
The Castafiore Emerald: Part 1 - S3-E7
Revealing mistake: When Castafiore locks her jewels in the drawer, she places the key inside the lock, but the key visibly surpasses the edge of the lock. (00:10:00)
Chosen answer: It is a pill that makes alcohol taste disgusting.