Deliberate mistake: Bare shoulders are forbidden in the Vatican premises, so the sexy dresses in the party would not be allowed.
Continuity mistake: In the scene in Berlin, the female member of the IMF team starts with no special glasses/goggles, then has them up on her head, then, just before the shooting begins, she puts them over her eyes, but later, while the shooting is still going on, they are back up on top of her head. Then they completely disappear. (00:17:50)
Continuity mistake: At the start as Ethan is handcuffed to the chair, the drop of blood underneath his nose constantly changes in size depending on the camera angle. This is also true when we cut back to that scene later in the film. (00:00:45)
Continuity mistake: The red wine stain on Davian's shirt is much redder when he enters the restroom than just after the wine is spilt on him. (00:51:00 - 00:52:20)
Answer: It's never explained. This is what's known in the film industry as a McGuffin, a term popularised by Alfred Hitchcock, which is an item that's crucial to the plot, but the exact nature of which is never revealed because it actually doesn't matter in the slightest.
Tailkinker ★