The popular interpretation of this story is that Adam and Eve gained the knowledge of good and evil when they ate from the tree. However, there are other rather interesting interpretations.
For example, in terms of the story God is the one who defines what is good and what is bad. Genesis mentions only one rule that God set down: not eating from that particular tree. Therefore, only one thing was defined as bad: eating from the tree.
Therefore, by this interpretation, it was not necessary to eat from the tree to gain the knowledge of good and evil; the tree's very existence provided that.
Thus we have an interpretation which doesn't actually contradict anything in Genesis, but also removes all of the mysticism from that particular section of the story; no magical gaining knowledge from a piece of fruit or anything like that.