In important test case to have would be [the infamous Turkish `i`](http://en.wikipedia.org/wiki/Dotted_and_dotless_I).

It's an issue in many languages/frameworks:
http://blogs.msdn.com/b/deeptanshuv/archive/2004/09/04/225720.aspx
http://haacked.com/archive/2012/07/05/turkish-i-problem-and-why-you-should-care.aspx/
https://groups.google.com/d/topic/golang-nuts/w8eZxT3dA48/discussion
http://stackoverflow.com/questions/16830570/qt-turkish-characters-case-conversion
http://wiki.tcl.tk/748
https://github.com/epeli/underscore.string/issues/252
https://github.com/nicolas-grekas/Patchwork-UTF8/pull/2
http://lotusnotus.com/lotusnotus_en.nsf/dx/dotless-i-tolowercase-and-touppercase-functions-use-responsibly.htm

I could add more, but basically every language/framework gets this wrong and [it has cost people's lives](http://gizmodo.com/382026/a-cellphones-missing-dot-kills-two-people-puts-three-more-in-jail).