Probably Pahlares holding on to heel hooks too long or Jones and his eye pokes. I also hate when guys are clearly overmatched and they resort to cheating like Conor or Anderson/Chael when he was holding on to the shorts.
And I know it's legit but part of me hated in the Aldo/Zombie fight where Zombie basically broke his arm and Aldo started going after the injury. Just seemed kinda scummy.
But going to the Jones eye pokes, I recall seeing an interview a long time ago where he hinted that him and Jackson went over the rule book and essentially admitted to using the pokes as part of their game plan. The idea is that you don't poke, you extend your fingers out and eventually a guy is going to run into your fingers. Since it's "unintentional", you'll never get a point taken. Once the guy is poked, he'll worry about it happening again and be compromised. That's about as dirty as you can get.
It's some straight Terry Silver "If a man can't see, he can't fight".