Montag committed at least one lapse prior to meeting Clarisse, about a year earlier when he encountered an old man in the park. In addition to listening to a poetry reading (not in and of itself a crime, but certainly considered aberrant behaviour) he failed to search the Professor despite being confident that he'd find a book of poetry on the man.
They had sat in the green soft light without saying a word for a moment and then Montag talked about the weather and then the old man
responded with a pale voice. It was a strange quiet meeting. The old
man admitted to being a retired English professor who had been thrown
out upon the world forty years ago when the last liberal arts college
shut for lack of students and patronage. His name was Faber, and when
he finally lost his fear of Montag, he talked in a cadenced voice,
looking at the sky and the trees and the green park, and when an hour
had passed he said something to Montag and Montag sensed it was a
Then the old man grew even more courageous and said something else and that was a poem, too. Faber held his hand over his left coat
pocket and spoke these words gently, and Montag knew if he reached
out, he might pull a book of poetry from the man’s coat. But he did
not reach out. His hands stayed on his knees, numbed and useless. “I
don’t talk things, sir,” said Faber. “I talk the meaning of things. I
sit here and know I’m alive.”
You may wish to note that senior Firemen, aware of the constant presence of temptation to which their men are subject, will often turn a blind eye to relatively severe transgressions (such as the taking and reading of the books that they're expected to burn).
“Well, then, what if a fireman accidentally, really not intending anything, takes a book home with him?”
Montag twitched. The open door looked at him with its great vacant eye.
“A natural error. Curiosity alone,” said Beatty. “We don’t get overanxious or mad. We let the fireman keep the book twenty-four hours. If he hasn’t burned it by then, we simply come burn it for him.”
“Of course.” Montag’s mouth was dry.