<bXY> is parsed as <b> if XY is a non-ascii character.
Examples (also included in the URL):
<b→> doesn't work! </b>
<bä> doesn't work! </b>
<boo> works fine </b>
URL: http://de.wikipedia.org/w/index.php?title=Benutzer:Church_of_emacs/Testseite&oldid=57126730&uselang=en
The parser misinterprets
*bar <s.baz@bar.com>
as
<li>bar <s></li>
Tested at https://test.wikipedia.org/w/index.php?title=Page479&diff=prev&oldid=220383
and originally seen at https://en.wikipedia.org/wiki/Special:Version/Credits/BetaFeatures where Siebrand's email-address is causing the issue.
@Legoktm noted: To reproduce you need to have $wgUseTidy = true;, but the parser doesn't actually have to call MWTidy::tidy()
See Also: T54022: <Sub-ID#1> is recognised as <sub> tag in <code> area