It had already been mentioned in another episode that Marcia & Cindy had the mumps. They were worried about getting the mumps in this episode, despite only being able to catch it once.
When Mike is playing the ukulele, his left hand does not change positions with the chord changes in the music.
Just before Millicent comes to the house to speak to Bobby, Greg and Peter are seen leaving for dates. During the sequence when Cindy spies Bobby and Millicent together, it is implied that Cindy is immediately going upstairs to tell her other siblings about what she saw. When Cindy goes upstairs, Greg is in his bedroom, not out on a date.