I can see how that could fail horribly when you have arbitrary Unicode data.

For instance, the German sharp s (ß) has an asymmetrical casemapping.

From the Unicode standard[0]:

>The German sharp s character has several complications in case mapping. Not only does its uppercase mapping expand in length, but its default case-pairings are asymmetrical. The default case mapping operations follow standard German orthography, which uses the string “SS” as the regular uppercase mapping for U+00DF ß latin small letter sharp s. In contrast, the alternate, single character uppercase form, U+1E9E latin capital letter sharp s, is intended for typographical representations of signage and uppercase titles, and in other environments where users require the sharp s to be preserved in uppercase. Overall, such usage is uncommon. Thus, when using the default Unicode casing operations, capital sharp s will lowercase to small sharp s, but not vice versa: small sharp s uppercases to “SS”, as shown in Figure 5-16. A tailored casing operation is needed in circumstances requiring small sharp s to uppercase to capital sharp s.

[0] http://www.unicode.org/versions/Unicode7.0.0/ch05.pdf#G21180