Continuity mistake: In Chapter 120, the main characters discover what they realize is an 8-by-8 Caesar box code. The first two times we see the letters of this code, the fourth sequence of four letters reads MFHA. The first letter, M, is a typo - it should actually be P. When the letters are grouped in an 8-by-8 grid in Chapter 121, this sequence has now changed to the correct PFHA. The correct letter P gives you the word "responsible"; the incorrect letter M would yield "resmonsible".
Factual error: The whole book is about a computer breaking a very strong code, and that for every code a large enough computer can be built to break it. That's simply not true: The "one time pad" cannot be broken by brute force, only by traditional stealing of the key.
Trivia: The cipher that Brown calls a "Caesar box cipher" is actually called a columnar transposition cipher. Julius Caesar did really invent ciphers, but the only one whose description has survived - and which to this day is called the "Caesar cipher" - is much simpler than the columnar transposition.