The client came in with a site that was intermittently unreachable and a previous developer who had stopped answering. The account had three domains stacked on top of each other with the primary document root pointing into a subfolder of another site.
Diagnosis took an afternoon: Cloudflare was set to Full (Strict) while the origin certificate had expired, producing 526 errors that looked random to the client because the CDN cached around them. Underneath that, the folder structure meant every WordPress update risked overwriting the wrong site.
I rebuilt the account structure, issued proper certificates, moved each domain into its own root, and set up automated off-server backups. Then I wrote it all down so the next person does not inherit a mystery.