At the end of the film, Horatio the dog jumps off the chair and runs out of the room not wearing anything. In the next shot of him going through the doggie door, he has on the outfit seen earlier in the picture, and then in the final shots he's not wearing anything again.
Detective Inspector Hook claims that the poison was ricin. Ricin is so lethal that the fumes alone from tiniest droplet will kill 22 averaged sized humans that are in an enclosed space like the average sized warehouse. Had this ultra-lethal poison been actually put in the carrot juice, everyone on the establishment would have been dead within 90 minutes. Even where Det Insp Hook says that it had been syringed into the can, by the end of the scene, all three characters would have been writhing in agony on the floor, certain to die swiftly simply for having been that close to the can.
At 00:25:00 when the camera zooms in on Rumbelow the shadow of the camera moves over Terry-Thomas.