What's your 'Oh no!' posture?

I'm sure I can't be the only one who groans inwardly when my yoga teacher announces we are going to work with Navasana, the Boat. It's my 'Oh no!' posture. I don't know why, but there's just something about it that I find really hard. My students know this, and yet still I teach it.

I do, of course, advocate that my students practise yoga at home, as well as in my classes; but I wonder how many of them only do the postures they like or that they can do easily. I speak from experience when I say it's really hard to make yourself do something you don't want to without the encouragement of a teacher. For instance, at home I hold plank for a lot less time than I do in a class, not because of my ego telling me I mustn't be the first person to fold, but because I know there is still something to be gained from staying with it.

What, then, is the posture you dread most? And is it because it's too difficult, too easy or just plain dull?