What was the Tree of the Knowledge of Good and Evil? There is a long discussion in the Talmud about this topic.
Some say it was the fig, since they used those fig-leaves to cover up and make clothes. Some say it was corn, which grew to immense heights and came already ground in those days according to legend - a child who can eat bread is able to speak so it must have been that. Still others say that it must have been the vine, since grapes are innocent until fermented and then cause us to act up and do bad things without understanding what we are doing until later. The apple, malus in Latin, is really a Christian superimposition on the story, because its Latin name sounds like the word in Latin for evil!
Yet what does the story really tell us? Should we have stayed in our innocent pre-fruity state, sat in the Garden of Eden and sustained by God alone? Perhaps the story is trying to teach us something else.
A person is not a full person until they have made a mistake. We learn from those mistakes, and despite the fact that our intellects allow us to learn theory, the practical effect of learning by trial and error is often the only thing which allows us to grow as human beings. A person who is terrified of making an error will never develop properly. That is why it is good to allow children to make mistakes.
God set up Adam and Eve. They were put in a garden where they had temptation before them day and night. Adam misinformed his wife, but he knew himself what he was doing, and he nonetheless went ahead and ate that fruit. If you put a small child in a room with chocolate and tell him or her not to eat it, you will be unsuprised to learn that if you close the door and come back five minutes later the chocolate is unlikely to be there. So Adam and Eve are not expected to keep the commandment concerning the Tree and its fruit. They are to be an example of transgression but at the same time an example of human frailty and that fact that we all make mistakes, no matter who we are. This is human nature.
The biggest mistake, however, is not to learn from the mistake and do better the next time we are tempted.
The story of Adam and Eve is one of Teshuva, repentance.