Give this a try:
http://www.mesacc.edu/~davpy35701/text/plato-no-err-willingly.pdf

There are quite a few examples of cases where people knowingly do what they believe to be bad for themselves, such as addictions, habits, and even simple instances of over indulgence, like eating.

As Socrates and Plato pointed out, this presents a dilemma for the moralist because producing moral people cannot be a simple matter of teaching morals. So this principle, that we knowingly do what is bad, provides the substance for Plato's attack on the sophists. The sophists held as a principle, that virtue is a form of knowledge. This validated the idea that virtue could be taught, and justified those sophists charging large sums of money to teach virtue. Socrates apprehended, probably through intuition, that these principles being taught by the sophists were faulty, and attacked this form of sophistry as being a type of scam. So he tried to uncovered exactly how the principles were faulty.

Plato latched onto this principle, that we knowingly do what is bad, as direct proof that virtue cannot be a form of knowledge, thereby undermining the premise of the sophists. You'll find that St. Augustine provides a much more thorough discussion of this dilemma. It has repercussions on one's understanding of free will, and the parallel dilemma of how a man's will can be free, yet God is omniscient.