I have encountered what must be a very frustrating event for anonymous editors. In our wiki we have the captcha activated, so that when anonymous editors are done editing, they fill out the captcha, and submit their edits.
The problem is, if they get the captcha wrong (happens frequently as they are not always clear), a nice notification does appear stating You have mistyped the anti-bot verification code. Please try again.
Then you click on "Go Back" and Anonymous is directed back to the wiki page they were editing, and all their edits are completely gone.
If you are aware that is going to happen, they can copy the text before filling out the captcha, but on a first time basis, it would be a complete surprise to see your edits missing after a wrong captcha entry.
I think it would be good to have their edits preserved after a wrong captcha entry.