In Lord of The Flies (by William Golding), there are several examples of mob mentality in ways both good and bad. One example is basically whenever one of the characters blow on the conch shell; as soon as the boys hear the "call of the conch", they all come over to where the sound is located. At the beginning of the book, a group of boys had been looking for water, but when they heard the conch, they went right to the source.
Another example is when the rest of the boys verses Piggy. When Piggy is embarrassed by the other boys, they all laugh at him. They call him names like "Fatty" and don't think that he would ever be a good chief because he's fat and acts like a rational adult. None of them (except occasionally Ralph) listen to him or sympathize with him.
The last example is when one of the boys, Roger, is pretending to be a pig that they just caught. The boys start out by pretending to attack him, but they end up hurting him and stabbing him with spears. When Roger says that they hurt him, the other boys just dismiss it as a game. Because of the boys started acting too savage (presumably Jack), the other boys followed and someone ended up getting hurt.