Many devices have proximity or ambient light sensors just under the surface that are hard to see but need to be exposed. For example, two cutouts near the earpiece of the iPhone may appear to be a mistake, but they are cutouts for the proximity and light sensor of the iPhone.

This proximity sensor deactivates the display when the iPhone is held near your face during a call - if it gets covered up, this feature will be disabled. That's why there's an extra hole cut in your skin. It's always good practice to review the entire skin sheet before installation to find all the possible cut holes that may need to be weeded out before installation.

If you install your skin and a speaker no longer works or your device is not responding as it should you may have missed a sensor cut out.