Apparently a mistake exists in Part 2 of the sculpture so it's plausible that there's more. From the Wikipedia article:

> There are also a few incorrect letters in the ciphertext which Sanborn has said were intentional, and a few letters near the beginning of the bottom half have been displaced from their normal positions, apparently intentionally... One of the lines of the tableau is one character too long, which Sanborn has indicated was accidental.

It's tough to imagine how to start such a problem when you can't even rely on the ciphertext being correct.