What does Percy see at the fruit stand that scares Grover so much in The Lightning Thief?

Percy sees a satyr at the fruit stand that scares Grover so much.

This is a pivotal moment in the book because it reveals that Grover is a satyr and he's been trying to protect Percy from the dangers of the world. The satyr at the fruit stand is a sign that the mythical world is real and that Percy is in danger.