in book i. of Spenser's Faërie Queene, is where St. George and Una encounter Error, who is slain by the knight. Una tries to persuade the Red Cross knight to leave the wood, but he is self-willed. Error, in the form of a serpent, attacks him, but the knight severs her head from her body. The idea is that when Piety will not listen to Una or Truth, it is sure to get into “Wandering Wood,” where Error will attack it, but if it listens then to Truth it will slay Error.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Wandering Wood from Fact Monster: