Some earlier printings of the 11th Edition of the Official Guide have errors that were fixed in later printings. The question you refer to above, for example, is printed correctly in my printing (the x is in the exponent), but I've read here and on other forums that it was printed incorrectly in earlier printings.

GMAC did issue a list of corrections to the Official Guide, which you can find here:

www.mba.com/NR/rdonlyres/B6424CFB-B6BA- ... l82006.pdf

Many people will not need this list; all of these errors are fixed in recent printings of the book. Apparently the correction to the number line diagram on page 154 is a particularly obvious mistake, so you can compare the diagram in your book with the diagram in the pdf file above; if they're different, then you need the above document, and if not, you probably don't.

All of that said, bizarrely the corrections pdf does not mention the problem with the 'x' in question 128. Still, that's fixed in later printings; I have no idea why this issue is not mentioned in the official list of corrections. And, there are still at least two errors in the printing I have, which I'll post about at some point. On the whole, however, I find fewer errors in the Official Guide than in other GMAT books.