After Sylvester puts Elmer to bed after singing him a lullaby only to wake him up again with a loud one-man band display, he escapes out a door with Elmer chasing him. When Sylvester goes out the door, the door knob is on the right side of the door, but when Elmer reaches the door, the door knob is on the left.
After being hit by the book 'Return of the Thin Man', Elmer closes the window, and the telephone rings. Elmer answers the telephone, and there is a stairway in the background, indicating Elmer is now on the first floor. But Elmer's bedroom is on the second floor. There is no indication that Elmer went downstairs at any point after getting out of bed.
When Sylvester initially spreads the tacks at the foot of the stairs, he scatters them right up to the stairs. But when Elmer comes down the stairs, there is now a wide gap between the stairs and the spread tacks.
When Sylvester sets up the grease on the stairs and tacks on the ground, Elmer rummages in his closet and pulls out a golf club to attack Sylvester. When Elmer goes back to get his shotgun, all the stuff he threw out of his closet when he got the golf club is gone.
When Sylvester wakes Elmer up by singing 'Some Sunday morning...', Elmer gets up and goes to the window. The table where he initially left his alarm clock now has books on it, and no clock.
After blowing up both himself and Sylvester, Elmer sees 18 singing Sylvesters float by - but cats are only supposed to have nine lives.