Debian Jessie using Gnome Classic. I scaled by 2 and had the same issue. I also closed the gnome tweak tool so when I re-opened it I was no longer still on the 'windows' section.
My solution was to click on a menu item that was visible, then use arrow keys to navigate the menu items until I reached 'windows', then I used ctrl + f (find text) and typed 'scale'. The scale item was brought into view and I was able to change scaling back to 1.0
