Seasonal Rot: Season 4 verges on it with a weak Detectives' season arc but is held up by the partnership of Cooper and Tang and how it collapses. Season 5 is generally considered weaker than the rest, however.
Shocking Swerve: the ending to episode 3 of season 3; we learn that Sherman's mother wasn't actually being raped when his front teeth got knocked out, the sex was consensual as she was doing it to pay off Ben's father's drug debts. I mean really, who saw that coming?
The last episode of Season 5: John Cooper comes back to his ex's house who complains about a neighbor's noisy generator keeping her up at night. He goes to confront the neighbor, who calls the cops. Said cops come and Cooper pulls out a gun, forcing the cops to shoot and kill him. One of the cops examines the corpse and immediately recognizes Cooper, pointing out that he's a highly decorated LAPD officer. This would turn out to be the final scene of the entire series.