The 'utf8' character set in MySQL does not support characters above
U+FFFF, which take up four bytes. (An example of such a character is
U+1D49E MATHEMATICAL SCRIPT CAPITAL C ("𝒞"), encoded as F0 9D 92 9E.)
Yet the web installer so prominently offers the "UTF-8" option, despite
this serious limitation. Perhaps MediaWiki should support the 'utf8mb4'
character set in MySQL 5.5 and above, in which that option is available.
Mailing list discussion that prompted me to file this bug: