Most module failures are not random. They follow the same patterns.
A module exposes too many inputs.
A caller passes unexpected values.
The plan succeeds, but the behavior becomes unclear.
A module hides provider configuration.
Resources become tied to invisible credentials.
Destroy operations fail when ownership is removed.
A module changes its structure.
State addresses shift.
Terraform plans replacement instead of continuity.
These are not edge cases.
They are design mistakes.
This lesson defines a checklist that prevents them.
Each item focuses on one boundary:
input contract, output exposure, provider ownership, dependency flow, and state identity.
When these boundaries remain stable, the module remains reusable.
When they drift, the module becomes fragile.
This is not about organizing files.
This is about controlling system behavior before it fails.