Most people think Terraform fails during plan or apply.
That is already too late.
Module sources are resolved during init. If Terraform cannot retrieve the module, the dependency graph never forms. No graph means no plan. No plan means nothing runs.
This is why module source types matter more than they appear.
A local path is fast but tightly coupled to your directory structure. A registry module separates location from version, which stabilizes reuse. A Git source embeds versioning into the URL, which introduces drift if you rely on branches. Archive sources shift distribution into object storage, moving failure into the installation phase.
Each choice moves where control lives. Each choice changes how drift enters your system.
This lesson isolates that mechanism so you can see the failure point before it happens.