I may just be too sensitive or something, but off the top of my head I can think of a handful of bits that I just didn't think were funny (because they were cruel).
I was watching an older show that brought it to mind. It was where they convince Steve Grillo to shave his head for a bunch of money (I think it was like 2,300 dollars). The bit didn't start out that way, it was all about them playing around with that canned hair stuff (made Scott the Engineer use it) which was funny because he was willing to go along with it. Then Howard made Grillo come in and basically berated him about how stupid his thinning hair looked until he finally used the spray. I think Grillo is a doofus, but I couldn't help but feel sorry for him, and then shaving it was the icing on the cake because he had transplant scars. It was just such an obvious self conscious attack from Stern cause he knew his hair was going.
Howard tends to act like an asshole most of the time, but there's a handful of bits where he just seems really vicious and awful, that one being one of them. He just WONT drop it and he gets this really weird look in his eye and keeps going till the person is totally defeated.
Another one that comes to mind is the HPE and the fish bit. It was amusing for a bit, but a lot of the time when Stern is harassing the wackpackers (no matter how irritating they can be) it just seems really cheap. They're so incredibly damaged and of course they knew what the signed up for, but I'm really not convinced these people can truly know what is going to come.
Same with the Bobo treasure trade bit (where Sal paints Bobo's face with his cock). I think Bobo is kind of a sick person (and we all know his IQ) and it just felt innately wrong.
Have you guys had any bits like this, past or present?