In a web browser (galeon, epiphany) go to a new page, then place the cursor on
the "Forward" button in the toolbar. The button is insensitive ("gray") and thus
you can't click it. Press alt-leftarrow to go back to the page from where you
came to this page. The "Forward" button become sensitive and you should be able
to click it to go back. It changes appearance to indicate this. The cursor is on
the button. Everything screams that "click mouse button 1 to go forward" should
work. It doesn't. The button does not react to mouse clicks until you move the
mouse out of the button and then back in. Then it works.

A similar thing happens occasionally with dialog boxes, which might open so that
the "OK" button (or "Cancel" or whatever you want to press) happens to show up
under the cursor, but clicking won't have any effect until you've moved the
mouse around a bit. This is not so easy to reproduce, however.

This may or may not be restricted to the GtkButton widgets.

Of course, this is not a big problem. It has also existed for quite a while