When Lily is having her flashback montage explaining why she felt the need to steal Druthers's baseball (specifically when Druthers is on the phone to his mother in his office), you can see that the ball is clearly already missing from its case before she was supposed to have stolen it.
When Barney knocks heavily on Marshall's teacher door, the whole wall starts rocking noticeably, revealing a prop wall was used.
At the beginning of this episode, Lily is sitting in Robin's apartment petting one of her dogs. However, in the previous episode it was stated that Lily is allergic to dogs.