Cognitive psychologists distinguish between automatic and controlled processes. Automatic processes are those that we can carry out without thinking (for example, driving home from work) and controlled processes are those that require attention. We use controlled processes when we do something new (for example, driving to a place we have been only once), when we are doing something very dangerous (walking on the edge of a cliff), or when we have to make a choice.
We can think of automatic/controlled as a continuous dimension, rather than a dichotomy. Every action we carry out has some automatic components and some controlled components. Many action errors occur from a failure of controlled processes. For example, let's suppose that I want to drive to the dry cleaner instead of going directly home from work. At the beginning, the two routes are identical, but there is a precise point in which I have to turn left instead of right. If I get distracted in that particular moment, I will probably continue on the usual way, and I will find myself home.
Donald Normal and Tim Shallice explain most action slips as failure of engaging controlled processes in crucial moments of the action sequence. If you are interested in action errors, you should check out the work of James Reason, the first psychologist to describe, collect, and analize action slips. A list of action slips can also be found in Don Norman (1981) "Categorization of Action Slips", Psychological Review.