Rob215 Simon said that an "Undo" feature will be coming soon. Chances are it will not be for 5 seconds, it will probably be the time that you last leave your mouse clicked.
How about the powder toy makes a Temp file with the saved game (will be like a stamp). The Powder Toy saves every 5 minutes (customizable to 1, 2, 5, 10, 15, etc) and replaces the previous one.
And, we can save manually, which will save in a different file.
Then, when we press Undo, we get snapshots of each stamp and are able to choose which one.