Just for clarity, I was just meaning in general, rather this particular incident, that if the ref deems it deliberate (by penalising it) a yellow should logically always follow.
There are of course instances where this doesn't happen, which suggests the ref either isn't convinced it's deliberate (in which case he shouldn’t penalise it) or is picking and choosing when to issue bookings, which is cheating/shitebaggery.
In the McHugh case, to me it looks ball to hand.
As has been stated, physics and biology prohibit you from having your hands by your side when slide-tackling.