What does the Helm error "another operation (install/upgrade/rollback) is in progress" usually mean?

Answer: The release's latest revision still has a pending status

Explanation

Helm writes each new revision with a pending status before it applies anything, and an upgrade refuses to start while the latest revision is pending-install, pending-upgrade or pending-rollback. When a Helm process dies mid-operation, that status stays behind and blocks every retry. Rolling back to the last good revision clears it.