Collateral damage is the unintended death, injuries, and damage to property of people who are not the intended target. Just as errant military operations may cause civilian casualties, sin wounds others.
Achan committed a secret sin, but his disobedience affected others. Joshua 7 tells how innocent soldiers died, the nation of Israel was defeated, and Achan's own family perished!
There is a gross myth today that says, "I am free to do whatever I want to do. I am not hurting anybody." The biblical record says otherwise.
Here's the challenge: the next time you are tempted to sin, remember your iniquity will hurt both you and the ones you love. Count the cost. Think again.