The loss put the skids under their recent momentum, built through four straight wins followed by a narrow away loss to North Queensland.
With Penrith having led early, a stray pass from skipper Matt Moylan allowed Souths winger Alex Johnston to run the length for the first of his five tries and what was the first of seven unanswered four-pointers for the Rabbitohs.
A shattered Moylan shouldered the blame in the post-match press conference. His coach Anthony Griffin said it took more than one stray pass to ruin a game.