Mary is going to say “Vatican Cameos” in a sing-song, malicious way right before she literally stabs John through the back while facing Sherlock.
There are multiple posts going on about this idea, so here’s my bet. Episode 3. Long after the divorce, Mary is found working freelance for the villain. She is given another chance to redeem herself. But again she proves to be heartless. This time she’s especially vindictive, excited to see Sherlock hurt the proper way: by watching his beloved die instead.
The bride stabbing Sir Eustace - the wife killing the husband - signals this. And how things come in threes… Private Bainbridge, Major Sholto, Captain Watson… three stab wounds for three military men. The sign of three, indeed. John’s gonna suffer but i bet he won’t be shot - He’s already been shot. He’s gonna get stabbed in the abdomen, to mirror the other ranking officers and to parallel the “cupid’s arrow” metaphor (“Mr Archer, on the count of three shoot Dr Watson” [ASIB]).