They matter when they distract you from the story.

It's fun to read what IMDb.com has to say about any movie. I usually check it out after I have seen a film, to avoid reading anything that will spoil it for me. I especially like to read the "goofs" section, which contains comments by viewers who have noticed inconsistencies like the one you described.

It's definitely a mixed blessing to have the kind of knowledge that enables you to spot mistakes in a movie. I remember when I first became aware of this. In 1965, as a college student, I saw "Lord Jim" with a fellow student who had grown up in India, and had traveled extensively in Southeast Asia. A major plot point of the movie involves a ship loaded with Muslim pilgrims en route to Mecca. My friend whispered to me, "Those [extras] are Sikhs." He could tell by the way they dressed. Later in the film, he said "That's a Cambodian village." I think the locale was supposed to be Indonesia or Malaysia. Obviously, most American (and, I suppose, British) viewers wouldn't know the difference.

Here's one that bugged me: In "The Reader" reading and writing are important to the plot. We see several close-ups of printed text and handwriting. It is all in English, although the story takes place in Germany and it logically should be German. Now, this is just artistic convention; it shouldn't matter to us any more than the fact that the actors speak English. But once you accept a convention like this, you expect it to be applied consistently. So it bothered me when I saw a police car marked "POLIZEI."