In my designers group we spend a lot of time discussing the kinds of details that cause knitters problems when working from patterns. We are all looking for the perfect instructional language that can't be misinterpreted by anyone.
Now stop laughing!
We try really hard to make patterns easy to follow. However every knitter brings a set of prior experiences to their knitting and that prior experience can determine how they interpret a given pattern. I've noticed that when the knitter can see a real error in the pattern, that they can quickly articulate what that error is and what the correction is. When the knitter can't explain or identify the error that often means they have misinterpreted a previous instruction, which is why the current instruction makes no sense to them. Often when a knitter tells me where the error is but can't resolve it, they turn out to be misdirecting me. I've learned through experience that the error actually occurred earlier on in the knitting and that they are simply looking at the wrong part of the pattern.
In these cases take a step back, pause, take a deep breath and go back through the previous instructions to see if you have missed a critical earlier step.