if you accidentally not correct something and you want to "go back", you can't.
It's not really undo. Going back does not have to be undo a correction. It could be go back to the last supposedly unspelled word.
Present since Ubuntu 12.04
Having some kind of button to go back to the previous misspelled word (regardless of the action taken on it when it was visited the first time) seems like a good idea to me. I can see how someone might have a long string of words to ignore or correct, and start clicking too fast and then later realize they made a mistake.
*** This bug has been marked as a duplicate of bug 135904 ***