Oops, I wish I hadn't done that!

For example, suppose I change the name of some function

And I change all the places that call it

But I make a typo

Or maybe I just screw up the log message

I could record a new commit with a sheepish correction:

          Subject: Oops, fix dumb error

Or I could record the same corrected tree, but with the last good commit as its parent:

I don't have to show anyone the bad commit

Eventually it will be thrown away