A shredder has a jam proof system with a thickness detector having a contact member which displaces as an article is inserted into the shredder and a resistance generating mechanism which provides a resistance force to the contact member, in response to its displacement. The greater the thickness of the article, the greater the resistance force realized. When a predetermined thickness is reached, there is a significant change in the resistance force. The resistance generating mechanism may include at least two spring mechanisms and provide feedback to the user that the inserted article may be too thick. In addition, the thickness detector may include a thickness sensor. The sensor may communicate with a controller to alert the user, and/or alter the operation of the shredder, in response to the thickness of the inserted article. For example, the controller may visually and/or audibly alert the user, or control shredder motor response.