In Mary's kitchen, after Dr. Neesam has left, the French dish on Mary's stove disappears and reappears between shots.
At the karaoke night, when John Cleese breaks off singing from the opera "Das Rheingold", he says, "Nobody leaves until Wotan has stolen the Ring from Alberich!" In fact, in the opera Wotan has already stolen the Ring when the excerpt being sung is heard.
When John Cleese pulls the knife out of his mouth, it wobbles, making it quite obvious that it is a rubber knife.