I read Fodor's Language of Thought over the summer. Towards the end of Chapter 1, he says psychological rules must have exceptions, since "even when the spirit is willing the flesh is often weak. There are always going to be behavioral lapses which are physiologically explicable but which are uninteresting from the point of view of psychological theory."
Like many cognitive scientists, I'm a big fan of Fodor. I love his theory of concepts put forth in Language of Thought. I even like his work when I disagree with it. This is a place I disagree: behavioral lapses are fascinating from the point of view of psychological theory.
There's two ways of interpreting this comment. One assumes dualism: there is an immaterial mind that tries to make the physical brain and body do what it wants. This theory is almost certainly wrong, but on this theory, cases in which the brain fails to do what the mind wants are interesting. Why would that happen? Are these errors random stochastic noise, or are there patterns in the failures?
If we assume that all behavior arises from the activity of the brain (probably the right theory), behavioral lapses are even more interesting. We have conscious parts of our mind/brain that make explicit decisions (I'm getting out of bed now; I won't eat that slice of chocolate cake; I'm going to smile when I open this present, no matter what it is), but it's clear to any owner of a consciousness that making a decision is one thing -- making it happen is another (there are more prosaic cases as well, in which we mean to say one word but a different one comes out).
Perhaps wires sometimes get crossed and our decision isn't transmitted to the relevant module of the brain. That seems like a pretty serious design flaw, so why hasn't evolution fixed it? Perhaps there are non-conscious parts of our brain that sometimes override our conscious decisions ... in which case, what is consciousness for (according to some, not for making decisions).
Psychological theory aside, I suspect most people would like to understand why, despite a willing spirit, their flesh is so often weak.