The final scene
I watched this on Turner Classic Movies with some discussion before and after with James L. Brooks, who had chosen this as one of his movies when he was a guest programmer.
I really have to agree with what Brooks said about the weakness of the final scene compared to the rest of the movie. The final scene, where Williams is an instructor in a class teaching undercover techniques, and then one student asks him if he's "the Ciello" and then says, "I have nothing to learn from you" and walks out, feels like it was slapped on at the end and doesn't add anything to the nuance, power and depth of the movie.
We already knew that other cops had issues with Ciello being an informant, an issue shown throughout the movie on many levels with great subtlety. I thought this final scene was pretty ham handed and not worthy of the superb screenwriting/storytelling that characterized the rest of the film. IMO, they should have come up with a better ending scene.
"Boy that was really exciting. I bet you're a big Lee Marvin fan aren't ya."