The status of Tom's mouse trap design changes from 'Reg. U.S. Pat. Off.' to 'patent pending'.
When Tom finishes drawing up the blueprint, he draws the mouse and writes "Mouse" underneath. When the blueprint mouse comes to life after Tom lies down on his bed, the word "Mouse" is suddenly underlined.
When Jerry's mouse hole is first seen, it's in a wooden section of the attic wall to the left of the blueprint. Once Tom has finished assembling his mousetrap, the mouse hole has inexplicably moved to a painted white wall on the other side of the attic.
The blueprint of Tom's mouse trap appears to be registered with the U.S. Patent Office - except Tom is still working on the design. The design can't be registered until it's complete.