When Jessica and Mitch are on the staircase the doors behind her are open. As the scene progresses the doors switch from open to closed then back to open. At no time did anyone touch or enter through the doors to cause them to change positions.
The main character carries around the cake like it's a piece of cardboard. In reality a 3 tier wedding cake can weigh 20 pounds. No way she would lift it or carry it the way she did without struggling.