Last modified: 2009-10-01 01:32:00 UTC
Create a page that includes say, a '<' character very near the beginning. Then select 'Delete' while using the Vector skin. The "Other/additional reason" form field will contain a literal '<' character in its value attribute, which is invalid XHTML. This makes the page unviewable on sites that use the application/xhtml+xml content type to serve MathML content.
This should be fixed in r56407. Make sure you have $wgHtml5 set to false if you want XHTML. Note that the variable documentation for it indicates that the option may be removed in the future (which would include removal of this fix).
Fix improved in r57182 to work for $wgWellFormedXml = $wgHtml5 = true case (which is the default config).