X and Z
I recently noticed on my iOS 5, jailbroken (but with no keyboard tweaks) iPad 3, that the virtual keyboard sometimes shows an undo button (in, at least, Mercury Pro browser). This takes the place of the Z and X keys. Note that the undo button is on the ABC keyboard, not the .?123 one.
Any clue as to what would this happen. I've been on places on the web typing and will realize I have no X or Y in portrait mode (it will appear in landscape).
Worse, when I'm touch typing (sort of), I may go to hit where the Z or X should be, and it will blow away everything.
Not sure if it's just Mercury Pro, but that is where I do more typing than other apps.