Grouped by symptom, never by module, because the person with the problem does not know which module has it. One hundred questions across ten pages, each answered with the causes in the order they usually turn out to be true.
These pages are found by symptom. Each question is answered with the causes in the order they usually turn out to be true, and the one check that tells them apart. The first thing to try for almost all of them is the same: look signed out, in a private window or with curl, because a signed-in admin sees unpublished records, uncached pages and placeholders that visitors never get. For jobs (called hooks before the rename to Orchestration) the first check is the batch log instead, for a store it is the API's own answer, and for performance it is the response headers and the time to first byte.
Twelve things that were changed, saved or uploaded and are not on the page, each with the causes in the order they usually turn out to be true, and the check that tells them apart.
Thirteen error messages and empty responses, what each one means on this platform, and how to resolve it, with the Smarty and Twig behaviour where the two engines differ.
Nine things authors report about areas, drafts, publishing, slugs and templates, each traced to the revision, the layout or the deploy that explains it.
Six questions about which engine a site runs, why a copied snippet fails, and where variables go across inheritance, includes and shared partials, with the answers verified by rendering.
Eight image symptoms on live sites, from the grey placeholder to the slow first visitor, each traced to a setting or a call and its fix.
Nine sign-in, session and access problems on member sites, each with its causes in the order they usually turn out to be true, and the check that tells them apart.
Eight store problems, from a cart that empties to a payment with no order, each with its causes in the order they usually turn out to be true and the API response that tells them apart.
Jobs were called hooks until the rename; the admin lists them under Customization › Orchestration. Six job symptoms, from a job that never fired to one that works on staging and not on production, each with its causes in the order they usually turn out to be true, and four answers about the rename.
Four performance symptoms, each with its causes in the order they usually turn out to be true and the measurement that tells them apart.
Twenty-one symptoms from integrations that read and write a Core dna site through its REST and GraphQL APIs, and from front ends that submit its forms: what each one means, what causes it, and the request that works.
On a development environment, every error page shows the stack trace with the template and line; on production the error is under Template Errors in the admin. The request path tells you which layer to suspect, and Your first page ends with the four checks for "nothing changed", in order.