It's rude to arrive late to your engagements.
Jane was invited to her best friend's wedding, and she is running late because she is waiting to finish downloading a movie.
Jane wants to make it to the wedding.
Jane stops the movie download and just leaves, arriving on time.
Jane waits to download the movie and then leaves, making her late.
No one notices anything wrong and everyone is happy.
Her friend notices and is disappointed in Jane.