Most connectors ideally consist of perfectly mated metal contacts. But when you see an enlarged image of what connectors actually look like you'll see that connecting surfaces show great irregularity - meaning that contact points are randomly scattered across the contact area of the connector. These uneven contact surfaces cause unpredictable contact resistance which ultimately leads to system malfunction.

On the mechanical side of things, surface coating damage along with the formation of cracks, dimples or pores can lead to serious connector problems. These could be caused by manufacturing defects, stress/differential pressure and/or growths from the contact pins, to name a few. On the chemical side, chemical reactions can lead to corrosive product layers that can alter the chemical behavior of the material and interrupt or weaken signal strength and eventually render the contact inoperable. Finally, on the electrical side, extended periods of use along with temperature variations can lead to contact resistance, a change in surface properties and joint welding. This is not an extensive list, it is merely a few examples of some of the things that can lead to connector failures.