One unreliable thing the narrator says is "its termination the feeble light did not enable us to see. proceed I said” this is unreliable because a normal friend wouldn't persist on keeping going in total darkness without a light. They are underground in the catacomb with no light.
Another instance where our narrator was when he said “You are a man to be missed” implying something may happen. why would a friend speak that way towards you
“You are a man to be missed”
The narrator is perceived unreliable In this section because the narrator says "he had given the explsive orders not to stir from the house." no friend orders another friend to stay unless its playful in witch this mood it is not. They are in Montresor's house and Montresor demands him to stay.