BreadcrumbList schema: what it is for and how it breaks
You added breadcrumb markup because a checklist said to, and you have no idea whether it is right.
What it actually does
It replaces the URL in a search result with a readable hierarchy, and it tells a machine where this page sits in your site's structure. The second is the durable benefit and it is the one nobody measures.
The three ways it breaks silently
Positions that are not sequential, so the trail is ambiguous. A trail that does not match the site's actual hierarchy, so it asserts a structure that does not exist. And a final item linking to itself when it should be the current page, which is common and harmless-looking.
Why it must be checked site-wide
A breadcrumb template that lost its middle level on one route is invisible on any single page. You only see it when every trail on the site is listed together, sorted, and read.
The breadcrumb replaces the URL in the result
This is the one schema type with a visible, reliable effect. Without it, your result shows a URL path. With it, it shows a readable trail. On long or ugly URLs that is a meaningful difference in how trustworthy the result looks, and it applies to every page on the site rather than to one page type.
The mistake that breaks it silently
The trail must match the page's real position and each item needs a resolvable URL and a position number in order. A trail that skips a level, points at a page that 404s, or repeats the same item will be dropped without an error anyone notices. Emit it from the same routing data that renders the page and it cannot drift.
Measured, not asserted
Every page on this site carries a breadcrumb trail, and a whole-site scan reads out each one, crumb by crumb, with positions checked for sequence. That check exists because the failure is undetectable page by page.
Free tool for this: Schema Inventory. No account, nothing uploaded.
Where this goes deeper
Every number on this page comes from one complete dataset: one product taken from zero to 100K+ users on search alone, with nothing spent on advertising. The full argument is Chapter 7 of the book. Five chapters are free to read.