The Bug:

The arm area of Chestplates have Z-fighting and clipping issues, where they clip through the torso repeatedly on each side because of the idle animation of Players. 
This wasn't much of an Issue when armor was all one solid color, but with the introduction of Armor Trims, this has become very noticeable and off-putting with certain trims.
It creates a "blinking" effect in motion.

A video of this in motion can be seen here, where it is much more noticeable than in pictures:
TrimClipping.mp4

How to Reproduce:

1. Put on Armor with a trim that is around the arms of a Chestplate
2. Watch it with the Player's idle animation
3. Take note as to how each side clips in and out, creating an almost "blinking" effect