Revealing mistake: When George Newton comes into the house and it is raining he has an umbrella in his hand. If you look outside his door underneath the balcony it's wet, but further down the path the ground is dry.
Revealing mistake: The scene with Beethoven in bed with George features a person in a dog suit. It's most obvious when the dog turns around in the sheets and has its mouth wide open. It's obviously not a real dog until the closer shots.
Revealing mistake: In the shot where Beethoven slobbers on the father's pants during the breakfast scene, you can see him pressing Beethoven's head against his pants, causing the "accident".
Revealing mistake: Towards the end while Vernon and Harvey are holding Beethoven the head of the dog is a dummy.
Answer: It appears he only gets pierced by the needles. The plungers aren't pressed, so he isn't injected with their contents.
Phaneron