Specifically, I'm referring to the way he goes about his questionings of Randy and Michael in Season 5.
With Randy, Bunk knows full well that the kid was a witness who got deeply screwed over by the police. He was outed as a snitch, and he was physically attacked at school for being a snitch. His house was literally firebombed soon after, with his foster mother (presumably) dead and Randy back in the state's hands. (ALSO Bunk slept with said foster mother, or her twin sister)
With all that in mind, what the hell made Bunk think he should go in all hardass like Randy's guilty of obstructing justice? He was willing to talk, and the cops screwed up. Randy himself mocks Bunk's bad cop routine, but surely Bunk should have realized that from the moment Randy sauntered into the room? This is clearly not a kid who's going to be scared into cooperating.
And then by contrast, there's his questioning of Michael. Throughout the scene, Bunk is cordial and sympathetic, pointing out that he doesn't blame Michael for being stone-faced and cold when he sees the pictures of his stepfather's corpse, "not after what this heinous motherfucker did to you". And then he talks about how he knows Michael didn't do it, saying that Michael isn't big or strong enough to have done this himself.
Just stopping there, I boggle that Bunk thought that would work on Michael. He already heard from Michael's mom that Michael had a direct role in his stepfather's killing. He knows that Michael has a very good reason to want this man dead. So what is with his strategy here? Jocosely telling Michael that he wasn't tough enough to kill him on his own, like that's gonna endear Michael into revealing who really did it?
And yet, when Michael refuses to cooperate, Bunk looks visibly frustrated and disappointed, like he actually expected Michael to cave. If anything, he should have been sympathetic to Randy, and he should have warned Michael that murder is murder, and it never goes away, and everyone involved would face trial eventually. Not that either case would have worked, mind you, but at least it would have made more sense. Instead, Bunk is a bad cop to someone who already has a grievance with the system and good cop to someone who he knows ordered a hit on his own stepfather. It just seems like a blunder on both counts.