This is a fairly common scenario. A user makes some significant changes to a post and then clicks Update. Realizing that a mistake was made, the user goes to Revisions. His previous post contents are there, along with the changes, BUT the Restore this Revision is inactive. You can’t restore an immediate Update! To do so, you must go back to the post, make another change, and Update. When you go to Revisions, your immediate change is shown again. Now, go to Previous Revision. There, you find the post where you made the initial mistake. NOW, go to the next Previous Revision. There is the original post from before you started making changes. The Restore previous Revision is active, and you can perform the restore.
This is a bug in WordPress! The immediate updated post should be available for restoration. It is not!