Spelling errors are one thing; what really gets to me is when a magazine prints a letter from a reader, containing really clear errors of fact, for example concerning license privileges.
Occassionally one sees this in a feature article. Implicitly this is presented as fact, when in reality it is no more than the writer's personal opinion (and he should have presented both sides of the argument). This is much worse, but even the letter mentioned earlier should not have got printed (well not without a clear correction from the editor).
There is SO much bull, incorrect information, occassionally disinformation (particularly concerning foreign licenses/ratings/registrations where so many people are grinding axes) going around GA that the magazines have a serious responsibility to get things right.
I have on occassions felt like writing in, but it's like these online forums (fora?). If one dived in every time something factually incorrect gets posted, one would spend hours a day online. As indeed some people do..... but those that do tend to contribute the least to knowledge.