Consider PIM, and assume we do the following modification. When a router has joined the shortest path tree to the source, it does not prune itself from the shared tree (RPT). Assume that the same forwarding rules apply: 

if you receive a mcast message from your parent on the SPT, you forward it to your children on both trees, the SPT and the RPT. If you receive it from the parent on the RPT, you forward it only to children on the RPT.

Show a scenario in which mcast packets may loop around forever (if we ignore the time to live field, of course)