When the assassin is removing the nun attire, palm trees are visible in the background.
Obvious Stunt Double during the "Nuns don't work on Sunday's" scene.
When Mac and Rick are "fishing" off the docked boat, it's obvious there is no line in their fishing poles.
The assassin fires a gun with a silencer in the balcony space of a church without being heard by the people below. Although often portrayed as whisper silent in TV and movies, guns with silencers are still very loud.
Nuns do not wear makeup, nor have plucked eyebrows. An effective assassin would have known that.