The people that inspect the rods at GLoomis have been doing so for 18-20 years now. They have seen more rod failures than all of us combined. This along with the facy machine that was developed to break rods will tell them quite a bit. A simpe ding to the graphite can cause a rod to fail.
The outer sheet of graphite may show no signs of impact. This is especially true with the painted rods. We use a fex agent in the paint to allow it to bend and flex so that it will not flake off. It can be easily dinged and return to the original shape. The inner sheet may be cracked making the blank weak in this area. The end result is a rod failure once it loads to that correct spot.
Typically a bad rod or a legitimate failure will look funnel shaped, or the rod will break into 3+ pieces. If the rod looks like it broke clean, with one side looking like a razor cut it, will typically be something that happened due to impact damage.
We don't just deny warranty claims because we fell like it. I know there have been some upset people because they couldn't physically see the damage. I have to place my faith in the people that inspect the rods. This is their job and they are the best at doing it.