GitHub Actions is the CI/CD backbone for millions of repositories. It's also the source of some of the most confusing, silent, and undocumented failure modes in modern DevOps. I've spent years debugging Actions workflows — first across 500+ repository migrations at an enterprise scale , then building agentic DevOps platforms that push Actions to its limits. This guide is the result: every error message I've collected, every silent failure I've traced, and every workaround that actually works. This is a reference guide, not a tutorial. Bookmark it. Search it when something breaks. Every section includes the actual error message (so you can Ctrl+F or Google it), the root cause, and the fix with copy-paste code. Quick Diagnosis Flowchart Start here: identify your failure category before diving into 65+ specific scenarios. Before diving into 65+ scenarios, start here: Workflow never appears in Actions tab? → YAML Syntax Issues or Trigger Problems Workflow runs but a step fails?…