Apparently touching the Radley house means death. In addition,
"A Negro would not pass the Radley Place at night, he would cut across to the sidewalk opposite and whistle as he walked. The Maycomb school grounds adjoined the back of the Radley lot; from the Radley chickenyard tall pecan trees shook their fruit into the schoolyard, but the nuts lay untouched by the children: Radley pecans would kill you. A baseball hit into the Radley yard was a lost ball and no questions asked."
Even adults felt the Radley house was off limits. They felt the Radley's, never going to church, were strange. They felt things went on in the house that were best not talked about.