Bug reports and feature requests relating to the MediaWiki software which powers Wikipedia should be filed on Wikimedia's bugzilla. Bugzilla is a bug reporting tool and organization system used by many software projects.
Bugs that are not directly related to the MediaWiki software should be discussed at Wikipedia:Village pump (technical). When in doubt, discuss issues at the village pump before filing a bug on Wikimedia's bugzilla.
Wikimedia's bugzilla was launched in August 2004 to replace the old MediaWiki bug tracker at SourceForge.