If you run a multiple monitor setup, when you click any of the menus in 2.5.6 they appear on the closest edge to your secondary monitor instead of on the monitor. I haven’t observed this behaviour in any previous versions and everyone in our office is running dual monitors, therefore this stops us from using 2.5.6.
http://www.igniterealtime.org/community/message/151360#151360 <-- someone else seeing the same issue in 2.5.4 many moons ago