The musical instructor tells the children that his instrument is a "tuba". Although that is technically true, it is really a Sousaphone -- an instrument created by J. W. Pepper at the request of "March King" John Philip Sousa. The instrument that Columbo plays, on the other hand, is a traditional tuba.
When Columbo does his analysis of the two sodas, one is full and the other half full. Columbo then leaves and Bradey returns to the fountain to dump the sodas in the sink. They are now completely empty with only a slight chocolate residue in them. Of particular interest is the fact that it is all one shot, which means the glasses were replaced while Columbo and Bradey are at the door. The originals were likely replaced with breakaway (sugar) glass to avoid possible injury to Fisher Stevens when they shatter.
Trivia: This episode contains a true rarity--a clear shot of Columbo's face as he reacts to the sight of an extremely grisly homicide. As befits a veteran officer, his reaction is restrained, but he does react.