Continuity mistake: When Barnaby is shot, the newspaper keeps moving away/towards him between shots.
Continuity mistake: When Barnaby is shot and lands on the ground, his arm is touching Mrs. Finch's door mat. When Mrs. Finch steps out of her flat, the body has moved away from the door mat (without being physically moved).
Continuity mistake: During the opera scene, the first shot of the bulletproof glass shows that there is a wire webbing within the glass. In subsequent shots, and when the glass shatters, there is no sign of the wire.
Chosen answer: He must have thought that Snowy had found the magnifying glass and was playing with it. Like most dogs would do with a toy.