Those Darn Finder Windows

The Finder has an Always open folders in a new window preference that, as the option suggests, ensures that opening a folder spawns a new window. You might think that disabling this setting would ensure that all folders are opened in the parent window; that is, alas, not the case.

(I'm posting this mostly because the following behavior bites me once in a while and by writing it down I'm more likely to remember the reason.)

When this setting is off, the Finder's behavior when opening a folder is also dependent upon the visibility of the window's toolbar. If the toolbar is visible, a folder is opened in the parent window; if the toolbar is not visible, either a new window will be spawned or—if the folder is already displayed in another window—the Finder will switch to another open window, perhaps even in a different space.