Jack trying and failing not to laugh his ass off in the car after is almost as funny. Even Exley eventually cracks up!
James Ellroy himself found the scene so funny that he was able to forgive the timeline fudging (the film is set before Johnny Stompanato and Lana Turner got together).
Bud tells Lynn's john to get lost, resulting in this funny exchange:
John: "Maybe I will. Maybe I won't."
Bud: "[flashes his badge] LAPD, shitbird. Get the fuck outta here or I'll call your wife to come get you!"
John: "[nervously gets clothes on and shuffles off and nods] Officer."
Bud: "[nods] Councilman."
"That how you ran the good cop/bad cop routine?"
Anything Mickey Cohen says in the book, especially when he's talking about his bulldog, "Mickey Junior".
Sid Hudgens is describing two gangsters, who are getting into their car:
Sid Hudgens: "...The sky's the limit!"
[One burst of machine gun fire later, both men are dead]
Sid Hudgens: "...Oh well."
There's just something darkly comical about the Movie Premiere Pot Bust scene.
Especially when Jack runs into Matt Reynolds again and is just waiting for Reynolds to recognize him, except Matt never realizes that this is the same guy who arrested him. Jack's reaction to Matt's "Have we met before?" is priceless.
Several references to husbands being discovered while cheating on their wife with prostitutes. Especially two involving the same city councilman:
The first time Budd goes to meet Lynn, this guy is coming out and Budd has to threaten to tell his wife to get him to leave quietly.
He prides himself on being all incorruptible, but is then blackmailed by Patchett into changing his mind after seeing himself in some rather compromising pictures.