[Computer_Genius]

After making a mistake instead of selecting undo from the edit drop-down menu press Ctrl +z
Which will quickly undo the mistake

[Goku]

Thanks for the tip Computer_Genius. I am sure someone will find it useful. For anyone interested, a more extensive list of keyboard shortcuts is available here.

-- Goku

[PCHero]

I do think by pressing ctrl-z on your keyboard is the quickest of undo editing on your wordprocessor, editor etc..

[JohnWill]

Alt-Bsp is a very common undo for many applications as well.