What is the "last lie" of the Mendacity Tree?

The last lie related to the Mendacity Tree is something that Faith comes to realize only when she is talking to Crock and Agatha in the cave. As she talks through the situation, trying to figure out a way out of the cave, she says that she believes the person who eats the fruit only sees the vision they already suspect to be the truth. This would be the ultimate trick of the Mendacity Tree: after having a person ruin themselves in search of some incredible revelation, they discover that the Mendacity Tree's knowledge is also a lie.