In a comment on my previous post, Julia of Twist Collective mentioned that they guarantee all their patterns. A brilliant idea!
Another comment made the wise point that a knitter should always search for Errata before beginning a project. This is one of the reasons I love the Internet. Google the name of the pattern and the magic work "errata" or "updates". Mistakes do happen - if they've been found, chances are they have been published online.
And this is why Ravelry is such a wonderful tool. First of all, it provides an easy way for designers to deliver an updated version of a pattern if a correction needs to be made. More importantly, however, the knitters themselves are an amazing source of information. Between the various forums and discussion groups, and each knitter's project notes, there is a wealth of information to be found on patterns.
This is particularly relevant when a pattern isn't wrong, but a knitter is just have difficulty with it. Whether it's simply a technique that's new to a particular knitter, or whether it's something that's not explained in sufficient detail, or perhaps just poorly or oddly written, the collective wisdom of the Ravelry community is an amazing tool.
In addition, you can often find the actual designer on Ravelry, and ask a direct question.
Patternfish, also, allows a designer to issue an updated version of a pattern to all purchasers. Knitty highlights pattern changes and fixes in pink.
Mistakes do happen - it's all about making it easy for knitters to find the fixes.