If you edit a post within an editing grace period and undo all your initial changes, the revision is silently destroyed.
That is, except when you use the rollback function, which is not how it should be IMO.
To test this, I vandalized this question of mine and immediately edited it out again, paying special attention to being character-perfect, whitespace and all. The revision was wiped and it disappeared from the front page.
Then, I tried it again, only this time I used the rollback link. I'd be lying if I said I expected it to work the same, because then I wouldn't have tried it, but it really should. Both revisions are still there even though the second one was within the 5 minute grace period and they cancel each other out.