Hermann writes to Felix: "... we must have an eye-to-eye monologue and get things settled."
And addresses us "I cannot recollect now if the 'monologue' was a slip or a joke."
The 1st one looks like a joke- oh Hermann is perverse. In the next chapter, "... I shook his right hand with my left..."
The 2nd one is definitely a joke- this time it's Nabokov's joke, and methinks he's laughing at Freud here. Freudian slips.