In chapter six of To Kill a Mockingbird, Jem, Scout and Dill almost get caught sneaking into the Radley property. Mr. Radley fires a gun, which scares the children, and as they are trying to leave, Jem's pants get caught on the fence that borders with the school yard. In his desperation to escape, he takes them off and leaves them there.
What spooked Jem afterwards was that his pants were eventually returned to him. They were mended and neatly folded. It is likely that "Boo" Radley did it, although the kids do not know this for sure.