http://en.wikipedia.beta.wmflabs.org/w/index.php?diff=118689&oldid=101159
This took a perfectly normal link like:
[[mw:User talk:Whatamidoing (WMF)|on Mediawiki]]
and turned it into something completely broken (single brackets instead of double, and space instead of pipe):
[mw:User talk:Whatamidoing (WMF) on Mediawiki]
Version: unspecified
Severity: major