"Categories ... are a useful way to get a handle on things you don't understand, as long as you don't get too attached to them and forget that things like to pop out of one category and into another, and that sometimes the whole category turns itself inside out and becomes something different." --Issola, Steven Brust
But the point is, they are only useful if you remember that you don't understand something because you have categorized it. You can't nail the category to the person and make them conform to it, and the fact that you don't understand them means that you're going to get things wrong. Some people actively resent the inaccuracies in the category we've assigned them. The movie "Talladega Nights" with Will Farrell is an example of an entire movie built on assertions about a category (NASCAR fans, drivers, and Southerners) which might be an illuminating study for those looking for category mistakes. That's where the movie drags: where you look at the screen and say, "Yeah, but it would be funnier if it were real." There are a number of miscues on what people are actually like in the movie, which is not a rarity in current cinema (those attempting to see "The Last Kiss" will find other examples), but it's still a little off-putting when it does.