Revealing mistake: When Sean shoots at Dennis the Bookseller's feet, puffs of sand do not erupt from the ground.
Continuity mistake: Near the beginning, a woman goes into an antique bookshop and hands the man who works there a battered book. At first, the book is the right way up, but when he puts it down it's suddenly upside down. When we see the book again, now it's the right way up.
Answer: Because, while everyone was scared and running for safety, he was the only one cool and calm. Plus, everyone was together, he was the only one alone and appeared out of nowhere.