Google announced on Google+ that they added an undo button in the change history screen within the AdWords advertising console.
The undo feature lets you revert back changes you made with the click of a single button.
The change history screen only shows you the changes over the past 30 days, so you can't undo past 30-days, which I guess should be fine for most active campaigns.
When you click View Change History in your Campaigns tab, you’ll see undo buttons with arrows that appear next to your list of changes. Find the row with the set of changes you want to undo and click the arrow button. You’ll be shown a verification that the change can be undone. When you are sure you want to proceed, click the Undo changes button.
You’ll be able to undo most, but not all change types. If the undo option is not available, you will not see an arrow button next to the listed change. Keep in mind that undoing changes will not undo the effects those changes had on your account.
For more details, see this AdWords help document.
Forum discussion at Google+.