According to Dr. Seuss, the film's creator and co-writer, one of the 150 boys vomited on the piano while filming. This caused a chain reaction and they were left with 150 vomiting boys. Dr. Seuss said that the film's reviews were similar.
According to producer Stanley Kramer, the film's budget would only allow him to hire 150 boys for the piano sequence, instead of the 500 boys he intended to use. When he threatened them with dismissal after they misbehaved, many of them stood up and cheered.
The words Bart recites while making the blood oath with Mr. Zabladowski are word-for-word from the Scout Law of the Boy Scouts of America. (Trustworthy, loyal, helpful, friendly, etc.)
When Bart and Mr. Zadlabowski are taken to the dungeon via elevator, there is no reference to a third floor dungeon by the Elevator Operator. The third stanza of the Elevator Operator's song was cut due to increasingly horrific lyrics referring to "household appliances". The complete deleted stanza went as follows: "Third floor dungeon/Household appliances/Spiked beds/Electric chairs/Gas chambers/ /Roasting pots/And scalping devices." (The reference to "gas chambers" was probably regarded as in bad taste since the film was made so soon after World War II and the Holocaust.)
When the boys are checking into Dr. T's piano prison, the kid immediately behind Bart identifies himself as Kim Charney, same name he used during brief career as child/teen actor.