Yes I have found mistakes in patterns. I email the author about it. Sometimes, the corrections are already on their web sites. Then, I write the correction in the book so that I don't make the same mistake again. I have published patterns myself so I know that errors can happen with the pattern going from person to person before it's published. We are all human and mistakes keep us humble.

Mistakes in books arent a big deal for me I can live with that. However what really ticks me off are quilt books that are sloppy. I have a few that are by "famous" quilters that are a very poorly done. One in particular, features blocks that are crooked, seams dont match and loose threads are scattered all over the place. If I am spending my hard earned money on these books, I want the quality content I expect from these quilters/authors. I have lost alot of respect for a few quilting stars, I get the impression its all about the money and no longer about the quilting when I see such poor quality.Is the art of proof reading gone by the way side? Getting off my soapbox now.

mistakes happen in books all the time- not just quilting books...sometimes a proof reader misses something- sometimes numbers get transposed- sometimes the printer changes something to make it fit- without having any idea what the text says---what difference it may make- mistakes happen- don't 'blame' the author- i would send an email to the publisher.
and why be (unwilling to sign up)? i don't understand that...if you received newletters & found they were not anything you found useful you can always (unsubscribe) but the author really has nothing to do with it anyway- it was more than likely exactly right when she sent it off to the publisher- they are the ones who can fix it- --- the author though-if notified can post a (fix) on her website-if she is informed...many authors/pattern designers have sections on their websites specifically for (errors found/fixes) related to their patterns.

Yes, and sometimes it is very clear, the math don't add up. Found it in several books.