Whenever I stumble on something that isn't quite accurate in a book, I feel cheated. I expect I'm too much of a pedant. Strangely, it seems worse in a work of fiction, which is a paradox. The fictional world is, after all, not real: but we have to believe that it is, and an inaccuracy can jolt the reader out of the make-believe.
There are so many ways in which the writer can be tripped up. Perhaps the finished work was not written in its final order, and the danger here is that common problem in filming, the continuity mistake. A character comes into the room, but he is already in the room according to the previous page. Somebody knows something which they could not possibly know because they were not there at the time and there has been no opportunity to tell them. Occasionally, characters even mysteriously change name mid-novel. I suspect that cutting and pasting plays a part in this kind of error. Pity the poor author, who has probably been looking at the text so much that s/he can no longer spot these things - but surely that's where the publisher's editor comes in?
There's another category of mistake too, the factually inaccurate. I've come across lots of examples of these. No names! Here are a few: a novel set in England in November, which begins with the characters sitting outside enjoying the sunset at 8 p.m.; a novel (by a famous author) in which a whole twist of the plot turns on someone forgetting to put the clock forward or back, but she has got it the wrong way round for the season (to be fair, that particular author did start life in the southern hemisphere - still, the action takes place in England!); a novel by another well-established author in which a son who lives in Devon proposes to run his parents into Portsmouth for shopping and tea - a round trip of about six hours and nearly three hundred miles by my estimate. Why do people get Plymouth and Portsmouth mixed up? Nobody who had any connection with either place would! It isn't necessary for writers to know every area they write about, of course, but for the reader who does know the place there's a risk of spoiling the effect if they get it wrong.
There's another category of inconsistency which we seem able to live with more easily, such as the fact that Ruth Rendell's Inspector Wexford seems to have been on the point of retirement ever since his first appearance nearly 50 years ago (that only jars a bit if you read the books straight through in one go). I gave up on another otherwise enjoyable series when a very dead character reappeared two books on though - I found it confusing, and a series presents a whole extra level of need for consistency if you are going to carry your fans along with you (or is it just me?)
One error I've seen very recently didn't spoil the willing suspension of disbelief in the same way, and it's one I'm sure would pass a lot of people by. A character had used Google to look for information, and noted a certain number of results. A second character did the same search, and got exactly the same results. Most unlikely, I longed to say to the author! Google search results are tailored to the searcher!
I do hope that being a librarian isn't putting me on the path to being unable to enjoy books ...