TYPO3 Core

Consider the page tree in a TYPO3 installation.

By clicking the top-left icon over it (Create new pages), the user can display additional, helpful buttons that aid in inserting new items into the page tree.
By clicking the same icon again, the buttons disappear.

These buttons should never be hidden (unless the user switches to the filter).

First of all, the buttons are hidden so well, new users (and even some seasoned ones) might not even know of their existence. Which is tragic, considering how helpful they are.

Second of all, there is no benefit in hiding the buttons. No screen real estate is gained from hiding them (as the bar they reside on stays visible at all times). I have a hard time coming up with a compelling argument why it could be useful to hide them.