The weirdest case is my Feral friend who can pull 10k on Grand Vizier... but I can still pull off him on trash if I'm not careful.
I've found my biggest problem with tank threat is AoE not single target. If I ever cleave/sweeping strikes off the bat then I'm going to have aggro on the X, then on the Skull shortly after. Live and learn I guess.
Note - I learned long, long ago that opening with Deadly Calm was a guarantee to pull aggro no matter who I ran with.