One of my pet peeves is gaps at the top of articles. (example: before and after)
The problem is exacerbated by a few things: 1) It's a relatively trivial change, so some would argue that it's not worth creating a new history entry just to fix the problem, 2) Many people don't seem to notice the problem (so even when they are editing the article to make major fixes, they don't fix this as well), and 3) even if editors do notice the problem, it can be sometimes very difficult to fix, and may require a good knowledge of wiki syntax.