It is rude to joke about a friends mom.
Joe and his friends are joking around and one of his friends gets a good crack in against Joe.
Joe wants to get his friend back.
Joe cracks a joke about his friend that is similar to what the two have joked about before.
Joe responds by making a crude joke about his friend's mom.
Everyone laughs at the joke.
Joe's friend is offended and gets upset with Joe.