Continuity mistake: Whenever young Riley is pulling the wagon around the house, Fear stops her at a white cord in an outlet. In the next shot, it is a black cord.
Continuity mistake: At the very beginning of the movie when Riley is born, the shot from behind shows the mother wearing the glasses. In the next shot the glasses disappear.
Continuity mistake: Bing Bong hurts his leg and cries candies. One with a grey wrapper vanishes in the next shot. (00:40:00)
Continuity mistake: On Riley's first day of school, she has a blue notebook on top of some other books on her desk. After her story about Minnesota, she sits down, where the notebook is now under another book.
Chosen answer: Joy must have seen the event. Since Joy was so focused on Riley's happiness and thought Sadness brought nothing to the table, she didn't realize that Riley needs to be sad at times to help her cope and move on to get to the happy times.