They establish that one ring by Lurch calls for Gomez the butler and two rings for Morticia the maid. But when Lurch first rings for Mortitia, he just is able to ring once (because he doesn't know his own strength, and thus he pulls the bell-cord too hard and yanks the rope right out of the ceiling) and yet Mortitia still appears, plus when Mother Lurch orders her son to call both of them and fire them, Gomez and Morticia appear instantly after just one ring.
During his "after-dinner drink" lesson, Lurch is initially holding the gigantic beverage-glass with a small amount of liquid in it, yet when he actually tips it back to drink from it, the glass is empty.