I don't work for Microsoft so I have no idea on why they code the behaviour the way they do.

But I would expect that the keyboard keys work relative to the 'active window'. When the document is open, the spelling pane is open and you've also been playing in the undo stack, it is possible that the spelling pane has lost focus and you need to click in that to allow the shortcut keys to work again.

IMHO, Undo is not completely reliable so it is generally better to make the right decisions in the first place and never have to undo. It may need a second press to firstly give the right window focus as per the earlier suggestion. Try clicking with your mouse so you know what has focus before using the keyboard shortcut.

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Andrew Lockton
Chrysalis Design, Melbourne Australia