As a little secret, "ctrl + y" does the same thing as "ctrl + shift + z" (redo), since many people have that association in muscle-memory from other applications. For the same reason, ctrl and command always behave identically, so that folks used to mac shortcuts or windows/linux shortcuts can use what feels natural, and backspace/delete do the same thing outside the context of editing text: clearing selections, deleting widgets, etc.
Another set of undocumented shortcuts are that pressing the number keys 1, 2, or 3 while using a drawing tool will open the modals for choosing stroke pattern, fill pattern, or brush shape, respectively, if for some reason (screen real estate constraints?) you prefer those to the toolbars.