When the RCA Victor dog leaves the billboard and chases the cat, he jumps into an exhaust pipe right after the cat jumps into it. However, it looks like he jumps behind the pipe, as the black oval of the opening stays black. The dog should have appeared over the black oval of the opening, and disappeared as he passed the front part of the pipe to appear as though he entered the pipe.
After the cat traps the dog in the exhaust pipe, the pipe suddenly shows much more detail, indicating the pipe was part of the background until that point.