All changes to a page are registered in a "page history", so any defacement can be replaced by an older version of the page. And, in general, recent changes to Wikipedia are automatically listed on a special page for that purpose.

Software robots automatically reverse obvious defacement immediately. Moreover, there are hundreds of people who spend a little time each day watching the list of recent changes on Wikipedia (see Wikipedia:Recent changes patrol). Any user interested in a particular page can add it to a personal "watchlist", which shows when a page is updated and gives the user a chance to check whether that update is a joke or a substantial contribution. Furthermore, many readers who pass by can correct vandalism or erroneous information.

So, the popular pages, which are the most likely to be defaced, are also those that receive the most attention from editors and readers, any of whom can 'revert' vandalism.

To stem a recurrent problem, an article can be temporarily protected from editing and/or user names and IP addresses can be blocked from editing.