In theory there is no difference between practice and theory, in practice, there is. While your computer case is designed to act as a Faraday cage (for EMI comp

In theory there is no difference between practice and theory, in practice, there is. While your computer case is designed to act as a Faraday cage (for EMI compliance reasons), this is only given when its new +some years old. Many opening/closings of the doors, disassembly/reassembly will wear out the conductive seals and eventually it wont perform as it was originally designed. Also, if the guy cut a big hole in the side to put a window in or something, that ruins the whole Faraday cage concept. Same goes if you put in a fan without a conductive filter closing the opening. ESD is in the GHz range and if you have any slots/holes in the case which are larger than the wavelength of the ESD, the fields will get inside.