We all make mistakes, right? Well, probably not this kind of mistake. Error is essentially a monster—half-serpent, half-woman—who embodies the problem of making mistakes. Redcrosse and Una in Book 1 literally wander into Error accidentally, and you might think that it's a bit unfair that just making a mistake gets you stuck fighting a monster.
But, hey, mistakes can have consequences and Redcrosse also show some lack of judgment in ending up with Error in the first place. Error lives in shady, dark forest, which is (in these stories, anyway) almost always the site of shady, dark activities. Error's cave his even more dark, and ever since Plato and his Allegory of the Cave, has been linked to ignorance and illusion.
Redcrosse was just so keen to get started being a big brave knight that he didn't stop and read the signs more carefully. Error's serpent-like qualities also anticipate the full-on Dragon Redcrosse will face at the end of the book.