Why would the hitman say "Michael Corleone says hello", if Frank Pentangeli was supposed to die?
As we all know, it's a botched attempt, because the cop walks in the bar, and everything goes to hell and a hand basket. This results in FP surviving, but hearing those last words, he thought Michael ordered the hit on him. This leads to FP deciding to testify against Michael, feeling betrayed.
It seemed to have made the story fit, as since the hit is botched, they've now turned FP on Michael. Was this planned? It seemed too convenient that the statement and the botched hit worked out so well.