Just found this out recently and I'm thinking it's got to do with 4.0.6. When you MD your pet on some bosses they bug out and are very easy to kill (Illidari Council went down to around 5% hp on pull and Felmyst didn't do any damage and just jumped around!)
on the illidari fight the pet was way back in a corner(MD pulled the warlock boss) and on felmyst it was just regular.
both of them bugged on the 2nd attempts.
also Brutallus doesn't hit with his meteor slash if you tank with a pet which makes the fight very easy.
I haven't tested all content yet but im sure it's bugged in more places, Enjoy!(If you are addicted to old raids like me =D)