The SaneCase extension automatic corrects case mistakes. For example if the page Test exists, and someone goes to TEST, they will be automatically redirected to Test with a 301.

This is really much more sane than the default case-sensitivity.

This implements the "Auto redirect option" from Case sensitivity of page names:

Automatically redirect to a page that has same spelling but different capitalization (have the computer do the disambiguation pages when a spelling doesn't match an existing page)

Negatives: Performance and possible search engine duplicate content penalties caused by MediaWiki's redirection mechanism.

My response to that:

• It's one very simple query, the performance hit should be almost unnoticeable for most sites.
• The user having to figure out the correct URL is often more of a performance hit, not to mention a huge usability hit. Plus these are the sort of responses that can be cached very well in Varnish or whatnot.
• A 301 "Moved Permanently" redirect should be fine for search engines.

Also see: https://www.mediawiki.org/wiki/Extension:SaneCase