Heck Tate tampered with evidence to make it appear that Ewell was killed by falling on his own knife. What piece of evidence did Heck remove from...

In the story, Heck Tate removed Scout's dress and bits of her costume from the crime scene.

While Atticus insisted that Jem must have been the one to stab Bob Ewell, Heck Tate disagreed. He argued that Jem only wanted to "get him and his sister safely home." The sheriff wanted to protect Boo Radley from being forced out into the spotlight for his role in saving the children's lives. Knowing how much the community's focus would have overwhelmed Boo, Heck Tate advised Atticus to let things stand as it looked:

There’s a black boy dead for no reason, and the man responsible for it’s dead. Let the dead bury the dead this time, Mr. Finch. Let the dead bury the dead.

Heck's removal of Scout's dress and bits of her costume was also his way of protecting the children from undue blame for something they could not have been responsible for: Bob Ewell's death.