How Pruning avoids loops in Protocol Independent Mulitcast (PIM- SM)?

I can not think of a scenario, how pruning avoid loops in PIM-SM when a router joins a SPT. I understood that it avoids duplicate messages, but how it avoids loops?

I want a scenario where the router joins the Shortest Path Tree (SPT) but never prunes the Shared Tree (RPT).

Can anyone give a brief idea how pruning avoid loops?

Thank you so much!