I've been asked just what constitutes an error and, by implication, what's going to stop me reading any further in a book.
Obvious errors are those of spelling and punctuation. Now, different people have different opinions on punctuation usage (how to punctuate ellipses is particularly fraught...), and different countries have different spellings: American "color" vs. British "colour" is an obvious example.
Grammar introduces a whole new area of confusion: while there are times when what's right and wrong are glaringly obvious, other things are more difficult to determine: there's an argument to say that split infinitives, for example, are perfectly allowable, despite the common perception that they are always wrong.
Finally, there's the author's voice to take into account. Sometimes what's correct isn't lyrical, readable, or interesting. Editors aim to make every book that passes through their hands as readable and fascinating as they can possibly be: so if by "correcting" the text an editor renders it impenetrable, then they're not doing the job properly.
All that waffle boils down to the answer that in many cases, there is not one right or wrong way.
HOWEVER (and you knew that was coming, right?) there must be consistency throughout a book or else the reader feels uncomfortable, and is alienated--which is exactly what we don't want to happen. So if rules are repeatedly broken in different ways, then yes, that's an error. Similarly, spelling mistakes are definable errors; and errors of punctuation are pretty easy to pick up, too.
To simplify things, each publishing house and imprint will have its own particular house style which sets out the rules that they expect all their books to follow.
I've edited books for both the UK market and the American market, and for perhaps twelve different imprints and publishing houses, which means that I've worked with a wide range of house styles, spelling rules, and authors. So I can be pretty flexible about which particular set of rules to follow. I'm looser about some of the grammar rules than others; I'm more pedantic about punctuation (I could go on at length about the correct use of the dash, but I won't here for risk of boring everyone even more than I have already).
So: I'll count as errors anything that's glaring; any inconsistencies of use; and any clear violations of grammar rules which can't be accounted for by the voice of the writer. And I'll be ruthless as far as spellings, punctuations, and grammar constructs go, while making allowances for regional variations and personal preferences. I will, however, be forgiving if some of the more complex rules are broken: most people just don't know how to punctuate ellipses, after all. I just wish they did.