When Chef Brockett makes the cake, he is following the recipe previously read by Mr.Rogers, but he fails to add the last ingredient - two eggs.
In the beginning when Fred comes in the door, the studio walls can be seen in the background. Same when Chef Brockett arrives and departs.
Just before Lady Aberlin sings It's an Ugly Day, Fred Rogers can be seen moving behind the Neighborhood wall just to the left of Corney's factory.