I've recently come across a UI where there is a two-level tree menu in the left column, as is common. The top level are categories, and the second level are the options. You click on the "[+]" button to expand it, after which it turns into a "[-]".
This is where it fails. The "minus" button, in the same style and decoration as the "plus" button, looks as if it will collapse the menu again when in fact it does nothing. This is exacerbated by the fact that with the default (first) category open, it looks as if there might be other categories that are not visible on the screen because it has a fixed height.