If you work in shared notebooks, it can be helpful to see what changes have been made by your collaborators. If it's not enabled, go to the Share menu in OneNote 2010 and turn off Hide Authors:

Hide Authors button

This will reveal vertical lines with initials next to any changes that have been made since you last reviewed the notebook:

Marked changes in OneNote

You can also review all changes by all collaborators by going back to the Share menu and clicking the Find by Author button:

Find by Author results

If there's a change you want to undo, click Page Versions to review past versions of a page and then select the one you want to roll back:

Page Versions

Suzanne