This came up when customizing an idea submission form. A custom field was removed inadvertently, and because it was a mistake, the admin wasn't sure which field was missing. An undo button would make a mistake quick to rectify.
I think an undo feature would be helpful in a lot of areas of our software: editing a campaign, editing an idea submission form, editing a funnel, changing customization settings, updating community colors. Probably more that I'm not even thinking about.