Continuity mistake: When Q is demonstrating the stun gas/explosive key ring, when Q holds the keyring it's very slim. However, when it is placed on the desk and emits the stun gas, in close up it is very thick. This is obviously to make such a slim device emit the gas (probably an under the table gag).
Revealing mistake: Saunders is not in the door-way when the bomb launching the glass door goes off. He actually jumps forward to get there in time to get caught.
Continuity mistake: When Bond pushes Pushkin back on the bed in the hotel room, Pushkin has a basket with a bottle of champagne in his left hand. It disappears between shots, and there is no sound of it falling to the floor.
Chosen answer: She could have been let go off screen once it was clear that Bond wasn't going to kill Pushkin, so they could formulate the plan in secret.
Captain Defenestrator