Occasionally the Wiki is hit by spammers trying to increase Google's ranking for their sites by adding links to them. These spammers may replace a page's content with dozens of links, or they may insert links into the existing text.
Fixing the changes
To fix changes, follow the little 'i' icon at the top of the page. You'll find a history of all versions and a revert button for each of them. Usually you can just revert to the previous version.
Preventing future attacks
Edit the LocalBadContent page. This page records a bunch of regex patterns that are applied to all of the links in a page; if a matching link is found when the page content is saved, the save will be disallowed and an error will be reported.
Remember that this page lists regular expressions, not strings, so the '.' character will match any character. If you're matching a domain such as foo.com, add foo\.com to the page.
See http://moinmoin.wikiwikiweb.de/AntiSpamGlobalSolution for more details.