The new piano was to be delivered to Mr. Rich's house at 666 Prospect Street, yet when the movers arrive, the address at the house is clearly 999. Mr. Poor's address was supposed to be 999 Prospect.
When Charlie Chaplin and Mack Swain remove the piano from the shop where they work, and pause outside to bicker, the shop window reflects a large crowd of bystanders who have stopped to watch the comedians perform for the cameras.