One thing that is commonly seen in hooves is bits of debris or necrotic tissue embedded in the hoof wall. The absolute most common place is in laid over bars, but the second is definitely the spots where there have been cracks or other penetrating injuries to the underside or topside of the hoof wall. It’s quite normal to see these types of problems in shod hooves because the nails that go in the hoof wall open up a spot for debris to get in and stay in. Some farriers/trimmers manage to get that all out, but some is missed then it can tend to build and stay in the hoof wall for a very long time.
Here is a great example of that:
As you can see on the right side of the picture, there is a band of debris stuck between the hoof wall and the sole. This kind of damage happens over time and without proper attention can just continue on up the hoof wall until it splits and cracks. Like a sliver in your skin, the hoof tends to just build up more hoof wall over top of it, which inadvertently makes the hoof wall very thick, but unfortunately weak as it’s not attached properly to the rest of the hoof.
The main resolution to such a problem is to use a hoof knife to scrape it all out until you get to clean hoof wall. The wall that is on the outside of that debris all needs to be removed though so that further debris can not get trapped in there. Until it is completely removed, there is always a path for debris and bacteria to make it’s way into the hoof wall to weaken it. Here are a couple of pictures of all of it removed and how much was required to remove to get there.
It was very important to leave the surrounding hoof wall at the appropriate levels to make sure the hoof can land properly. There may be cases where you can’t get it all in one go and have to be patient to just pick away at the edges of it all until the hoof grows down a bit more. This should be a judgement call made by a professional of course so as not to lame the horse.