Don't let me find out you were stirring about when you're not supposed to be.
He's not even dressed up like a jester
Montressor intends to kill Fortunado and that makes him not lucky to meet Montressor's acquaintance.
It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
Montressor speaks to the servants telling them he'll be gone till the morning and doesn't want them moving around.
What's your family's motto again?
Even though Fortunado dressed as a jester (something that is meant to be funny), Montressor is the one being laughed at by people at the carnival.
*Cough* *Cough* *Cough*
This is stated at the beginning of the story by Montressor because it shows his dark side since Fortunado wronged. This foreshadows the general idea that nothing good is going to go Fortunado's way throughout the stor.
This motto strengthens the thought of what pans out for Fortunado even though he can't infer that.
Nemo me impune lacessit.
Fortunado says that his coughing fit won't kill him but what he does not know is what he just said foreshadows the sickening event about to be taken place.