Normally, this isn't much of a problem. One trusts ordinary sites not to link to such places. On most sites publically editable content (which could link to bad sites) is separated from the main site content, so it is clear who is responsible.
On a wiki, however, anyone can edit (almost) any text on the site, including older trusted text. People learn to deal with the problems of trust in a wiki, and should be skeptical of strange claims on a page. For instance, if the CliffordAdams page claims that Cliff has always hated Perl (and that FORTRAN is the One True Scripting Language), then someone else has probably been having some fun.
One can enjoy the strange insecurities of a wiki, and even laugh at weird edits made by third parties. The fun stops when editors can cause immediate serious damage, especially if the victims didn't think they were at risk.
On some wikis (which allow arbitrary HTML), a single malicious person could damage several systems (before they are blocked from editing). On other wikis which allow arbitrarily-named remote links, "CliffordAdams" or "RecentChanges" could be a link to a bad/damaging site. On wikis which allow InterWiki editing, "MeatBall:RecentChanges" could be bad. Finally, on most sites (including here), "http://www.some.bad.site.example/mystuff" could be a bad link, but at least one knows where one is going.