How can you understand that a slide callipers has an instrumental error?

How can you understand that a slide callipers has an instrumental error?

The main scale of the slide callipers is made of a graduated rectangular still plate. A metal jaw is fixed at the starting end of the scale, that is, at the end marked '0' of the main scale. When the jaw of the vernier scale touches the jaw of the main scale then the zero of the main scale should coincide with the zero of the vernier scale. If two zeros are not marched, then we can understand that the slide callipers has an instrumental error.